A funny thing happened to me on the way back to my flat the other day. I was nearing the end of my last big run - the final one before the marathon. I'd been trotting for around 2 hrs 40 mins, about 17 miles, and I was feeling fine.

My ankles were still attached to my body, I'd avoided all the normal obstacles on my routine route (pubs, sandwich shops), and I was actually enjoying myself. I knew I was getting close to home when, quite inexplicably, I started thinking about shopping for food.

I saw myself clearly limping down the aisles at Waitrose buying all the things I shouldn't buy. Lots of oily, fatty, inappropriate things: crisps, biscuits, cheese, more cheese, microwave chips, vegetable samosas. I was planning a big lunch and I knew it wasn't far away.

As I woke up from this fantasy feast I realised things had started to go wrong with my running. My legs felt heavy, my knees were sore, people who once provided interesting furniture on the pavements were now annoying hazards. The commentary from my Walkman was irritating me. I was looking at my watch every minute, I felt as if I was treading ground - going slowly nowhere. I started getting worried.

I knew I didn't have long to go to lunch. But would I make it? Would someone else eat it before I got there? I wanted to stop. It was no longer any fun.

Looking back now from the comfort of my crisp-strewn sofa I realise I had hit my wall. Perhaps not as large as the one I may come across on marathon day but all the same a mini-wall. The nearer I got to my destination - home and lunch - the harder it became to get there. It seemed to come out of nowhere. One minute happy, the next a wreck.

The experience has been a bit of a blow. I hadn't had a shopping fantasy before - and the last place I wanted it was in Camden Town wearing a pair of shorts. I'm also slightly concerned about what will happen on the big day.

Happily, there are things I can do to stave off this madness. Mainly I have to watch what I eat and drink over the next 10 days. I need to get hydrated - lots of water every day. I also need to eat the foods that will provide me with the energy I need: pasta and nuts, raisins and bananas.

On the morning of the race I need to be up early, eat small, drink large. And I need a secret weapon. So I'm packing a bag of jellybeans in my shorts. They come highly recommended and will provide me with little carbohydrate doses through the run, keeping the wall always one step ahead.