I hate Facebook. I stick with it because it’s the only way to keep in touch with some of my family and friends, and because my running club, Clapham Chasers, use it in lieu of a forum so it’s the only way to get hold of news and plan trail runs. But wherever possible I steer well clear of the bloody thing. Photos of babies, posts about guardian angels “and if you don’t share in five minutes your angel will die”, those bastard water bucket challenges. BAH.

But then, one day recently it came useful.

I knew about the Royal Parks Half Marathon from colleagues who had done it in previous years, and I knew you usually needed to raise money for a charity to get in, but I had no idea there was a 50k course until the Runners Need page threw up a post saying they had a number of places to give away. All you had to do to be in with a chance of winning one was click on a link and enter some details. I’d sell my own mother for a shiny doubloon, so I gave them everything – address, phone number, daily schedule and list of fears – and forgot all about it.

Then, one day about three weeks out, my mobile phone rang. Being the charming, antisocial sort that I am (and working in a concrete bunker with no signal) I let the call go to voicemail, but when I listened back it was someone from Runners Need called Kirsty saying that a place had become available and did I still want it? Within a minute I was jigging around in the fire exit, trying to get enough signal and yelling “YES PLEASE ME PLEASE I’LL TAKE IT”.

It’s a London race so it should be easy enough to get to, I thought, and from the finish in Bushy Park it’s a single train home. Lovely stuff. Yeah, not so much – thank you TfL. Two hours and three buses later I finally reached the Runners Need stall to get a group photo and pick up my race vest, and it would be a twenty minute hobble to the nearest station at the end followed by a replacement bus service. If I had known quite how bad the transport situation would be I’d have genuinely considered running to and from the race.

Both the half marathon and ultra courses are designed to take in the greenest areas of London, starting in Hyde Park, skirting Green Park and St James’ Park and picking up the river at Westminster. There a nice sweeping stretch going east as far as Blackfriars, where the half marathoners turn back and the ultra runners cross the bridge, turning west on the other side to follow the Thames Path pretty much the whole way home. The three parks and Blackfriars section happens to be the route I run with my work running club, the Thames path from Waterloo to Wandsworth part of my run home and everything west of Wandsworth my favourite easy Sunday run route, so I pretty much knew the whole course already. This is my territory, I thought.

Maybe because I hate being anything less than 2 hours early to a race start, maybe because buses freak me out, maybe I’m just a spanner – but I found the race village in Hyde Park confusing and difficult to navigate and got myself into a right flap. I walked the length and breadth twice following conflicting instructions for the bag drop, until it became clear that the ultra runners were operating out of a separate tent for pretty much everything. With all the banners advertising the half marathon and its innumerable sponsors, and only the odd arrow pointing on the general direction of the 50k start, I began to worry that I’d come to the wrong park. It wouldn’t be the first time.

Eventually though I lined up among the other 500 or so runners, with two empty water bottles I’d forgotten to fill in my flap, and took off towards the river. The course is relatively flat apart from a couple of bridge crossings and an incline in Richmond Park, so I let myself settle into a comfortable pace slightly faster than I would normally risk, and immediately felt calmer.

The first 5k went in no time whatsoever, so I took advantage of the first checkpoint to stop and fill up my water bottles. By this point we had crossed Westminster Bridge, turned a hairpin and come back across it again, and I could see the National Theatre across the water waiting for me to pass by. Being my workplace, I see that huge angular behemoth almost every day; its washed out grey concrete usually looks stern and humourless under the crisp LEDs that usually illuminate it. Under the soft warm Sunday morning haze though, it looked beautiful.

I fell in step with a chap called Tanbir aiming for around 5 hours in his first ultra distance, and still feeling comfortable I ended up spending a good 25k with him, chatting all the way. We trotted through Battersea Park, hugged the south bank of the river around Wandsworth, fired up the old glutes with two bridge climbs in quick succession through Putney, and gossiped our way past Hammersmith, separating only temporarily while he stopped for a comfort break. We were looking at Barnes Bridge just after mile 15 before either of us even noticed that familiar tightness in our hamstrings, and like that the easy chat turned to awkward small talk, skirting around the sharp realisation that we had hit halfway.

I hate halfway. Up to halfway I’m not thinking about what’s left to do, I’m just enjoying myself and letting the miles fall away. After halfway, however tired you are, you know you’ve got less left to do than you’ve already done. But at halfway, neither thought consoles you. As though I’d had been gently stretching an elastic band and suddenly let it TWANG and fly from my grasp, my muscles suddenly seized up like quick drying cement and I felt heavy.

At this point, we started to take the run in small chunks. Keep going until we hit the 30k checkpoint. Then we can stretch. Then push on until the marathon. Then another stretch. Then one final push and we’d be done. Little chunks. Easy as that. Every time I allowed myself to throttle back and slow down the cement began to harden around my muscles. As long as I kept moving they couldn’t set. With the ground becoming more gravel and less pavement, we ploughed unsteadily on.

The 30k checkpoint – generously stocked with crisps, Percy Pigs and Lucozade jelly beans – came just in time, but after a quick pause to inhale some junk food we realised we needed to push on. Tanbir was suffering much more than me though, and eventually he let me go on ahead while I still could, assuming that we’d catch each other eventually. I should have been appreciating the stunning surroundings, particularly as I approached Richmond Park, but I missed the company so switched on my audiobook.

Every day’s a schoolday, they say. I learned something just before 40k. I learned that audiobooks are brilliant for long runs, but never underestimate the fragility of your disposition when exhausted, and never listen to graphic or upsetting stories when you’re 40k into a run through woodland. To be fair, I thought that Haruki Murakami would be a relatively safe bet, but after three minutes of a graphic and detailed account of a man being skinned alive, I nearly threw up into the Thames. I don’t know why, but it immediately made me think of my boyfriend Andy, and oh god what would I do if that had happened to him, as if something like that ever would, and how much I wished he was with me, and then I nearly threw up again. And then I switched to music.

For me, a good litmus test of how I’m holding up mentally is by testing my mental arithmetic skills – converting my pace in minutes per mile to minutes per kilometre for instance – and quite often, even when I think I’m ok I find myself unable to remember what number comes after 12 or how to multiply by 20. I stuck to Tanbir’s plan – to keep going until the marathon mark and then allow myself a walk break – but I forgot that I was meant to be looking for a marker saying 42, not one saying 26, so I’d convinced myself I still had a while to go. When the marathon marker did eventually sneak out from behind some bushes, I was stunned. I checked my watch – 4 hours 11 minutes. A full hour faster than my time at Brighton Marathon back in April.

Relieved to finish the first 26 miles I slowed down to walk and ring my equally surprised boyfriend – who hadn’t been expecting a call from me for another hour – but it wasn’t until after the race was done and dusted that I realised what I’d achieved. At that point I just wanted to hear his voice and be reassured that a Mongolian warrior hadn’t skinned him alive in my absence. I did however have the presence of mind to work out that a sub 5 hour finish was on the cards. Love and kisses, phone back into my belt pack, game face on.

Despite much of the first half being run on pavement and footpath, the towpath that replaced it was getting gradually more and more gravelly and the stones underfoot bigger and sharper. My light soled road shoes had been the right call to begin with but my toe joints were now bruised and painful, and every time I landed I had to hope it wasn’t on another pebble. I listened to a playlist of ska punk tunes to keep up my spirits and sped up in the hope that it would mean less time on my feet.

The final section through Bushy Park is somewhat easier underfoot, but without the Thames as a guide it’s difficult to judge just how far you are from the end. Every time I pushed on, I turned a corner and found another stretch. Every time I slackened the pace, I saw a marshal urging me on. And I had no idea how close I was to a 5 hour finish.

And then, there it was – the home stretch. The marshal directing us in was shouting that we were nearly there, but I’d heard that said so much in the previous half hour I nearly didn’t believe her until I saw the arch. Uneven ground, soft mud and stones be damned, I lifted my knees for a sprint finish and threw myself across the timing mat, shaking and sobbing. My watch told me I was within a minute of the 5 hour mark, but I didn’t believe it until a moment later a text message popped up on my phone to confirm it – impressively quick result reporting, given that I was still gasping for breath. 4 hours, 59 minutes and 18 seconds. I had beaten my marathon PB with my 50k time.

I got my grubby little magpie fingers on my medal: a wooden one shaped like a leaf, which I thought Andy would like since it wouldn’t be as noisy at the metal ones. The goody bag was deeply impressive too – a little canvas bag full to bursting with a packet of spicy chickpea snacks, a sachet of porridge and a box of single serving cereal, a peaked Buff style headband from Crewroom, a set of short stories commissioned by and written about the Parks, deodorant, a bag of sweeties, a water bottle and a partridge in a pear tree. In fact the bag was so full of stuff I somehow lost my voucher for a free hot meal to its Mary Poppins carpet bag interior, but after being shaken and run ragged for five hours my stomach wasn’t up to much anyway.

Apart from the kerfuffle at the start – and despite the best efforts of TfL – it was a well organised and logistically sound race. The checkpoints were generously stocked with treats and snacks, sponsored as they were by Marks and Spencer (hence the amazing goody bag at the end) and the field was of a manageable size so it never felt like a scramble. Even better, the support was fantastic considering it was a long point to point course; although it wasn’t exactly a throbbing party atmosphere and quite often race participants were outnumbered five to one by Sunday joggers, everyone we passed from Putney to Bushy Park seemed to know what we were doing and cheered us on.

For a race I wasn’t even expecting to do, it was something of a triumph. I’m right at home with the 50k distance, and for the first time during a long run I barely walked at all. The Thames is often my running guide, and if I’d been asked to design a 50k course this would basically be it – no further East than Blackfriars and plenty of the South bank.

When I finally made it home I just sat on the sofa staring at my Garmin readouts. If someone had challenged me to beat my marathon time at the marathon point I couldn’t have done it. If someone had challenged me to beat my marathon time with my 50k time I’d have laughed. But now, there’s a whole new set of possibilities.