I woke up at 4.30am in a small bus stop in rural Anglesey. I was freezing cold and my legs were seized up, yet I still had 50 miles of a 107-mile cycle race to run. Why? Good question.

I ran the Norfolk ultra marathon (62 miles) last year and got a lot out of it. I had forgotten most of the bad bits when I decided to run the 108-mile Tour De Mon bike race at the start of this month. A friend of mine who has a house in Anglesey had sent me an ad for the race after finding out I'd bought a road bike for my commute. But I'd made a new year's resolution that I would attempt to run 100 miles in one go in 2013, so I thought this would be the perfect opportunity. I rang my girlfriend. She thought it was a daft idea and that I'd be better doing a marshalled ultra marathon. Work colleagues concurred. Alas, this just seemed to encourage me. I phoned the race director. He initially laughed at the idea, but said that as I was doing it to raise money for a good cause, he'd happily give me a place.

I set off at 5.45pm. The weather was fine and I felt strong. But the first marathon marker seemed to take an age to come. I had to deal with millions of flies, a slight detour on to a shingle beach (about 1.5 miles) but that first benchmark arrived just as night fell. In a high-viz jacket and with a torch in each hand, I jogged onuntil mile 40 where I met a friend at Menai Bridge.

One of my major concerns prior to the race had been finding my way. I'd scoured the route on Google maps and wrote out directions to follow as I ran, but there was no need: the Tour De Mon is a professional setup and there were regular arrows on signposts and lampposts and even on the road to show the way.

My friend stayed with me for the next four and a half hours, rolling beside me on his bike while singing random verses of hip-hop songs. He timed my 20-minute running, five-minute walking routine with a loud alarm. I averaged 5mph, ending with my only stoppage in the bus shelter, where I slept for 20 minutes.

Waking at 4,30am, freezing cold and with seized-up legs, I couldn't run on. Every step hurt. But I phoned my girlfriend and she talked to me for about 40 minutes until the sun started to rise, by which time my youngest boy had woken, I could hear his voice in the background and boom ... I was running again.

I knew I had support around mile 70. If I could make it to there I'd have my girlfriend's mum to help me through the next marathon. And she did, breaking it down into 4x10k runs. The first took 60 mins, the second 75, , the third 60 (all up hill) and the fourth 70 – at which point I felt great, quite possibly because of the cheese-and-crisp sandwich I had at the final drink station.

The cyclists, meanwhile, had set off at 10am, so I was expecting them to start flying past me early afternoon. A few of them had obviously heard that I was running their route and shouted words of encouragement as they flew past, though for every "Well done, mate, keep going" there was a "You must be mad" or "You nutter".

The last half marathon was down to me alone. I put The Railway Children (the charity I was raising money for) running vest on and set off knowing that this was all mental now: my legs were in bits, my hips were aching and even my upper body muscles were killing me – I'd have to be headstrong to finish.

Marriott's son helps him cross the line. Photograph: Rowan Marriott

With five miles to go, I passed the house that I was staying in. Family and friends were outside to shout encouragement. Running past my parked car was torture: I would happily have stopped there, having already run more than 100 miles. But everyone said I had to finish it – and I'm so glad they did. The last few miles were tough: hilly and never ending. With about a mile to go I figured I had about two runs left in me and I saved the last one for when I saw the sea. The finish line was on the seafront at Holyhead and on my approach my seven-year-old boy joined me and ran the last 100 metres with everyone clapping and cheering, a moment I'll never forget.

I finished the 107 miles in 23hr 5min, within my target of 24hr – and I even beat a couple of the cyclists.

An unbelievable experience and one that I'm still coming to terms with. I've had so much support and interest its blown me away and best of all, I've raised more than £1,350 for The Railway Children.

• Donate to The Railway Children on Rowan's Justgiving page