One week into my training and I have a problem. My gluteus medius isn't working properly. Consequently my tensor fasciae latae (not a coffee) is being put under a bit of pressure. It's too tight and I'm using it too much. The result: I'm running like Norman Wisdom.

For the first time in my life I've been to see a physiotherapist. I've been told it's the best thing to do - sort out your problems before they sort you out. And so a woman named Angela Bell inspects my body, which is still the four mince pies short of slim. I explain my marathon schedule and I tell her I have a sore knee which has haunted me since school.

I arrive somewhat sceptical, but 45 minutes and a brief Latin lesson later, I am a convert. Bell first spots my pronated feet - basically the inward step of each foot is slightly turning in. That's probably not helping my knee.

Then I'm encouraged to show off my running technique on a treadmill. Bell stands behind me, puts her hands on my bum and squeezes my buttocks as I run.

It's then that she breaks the bad news. I have a lazy bum. My gluteus medius seems to have forgotten how to work. Normally the action of raising your leg and placing it forward is aided by the contraction of this small muscle at the top of each side of your bottom - where your wallet would be when wearing jeans. Because it's not working my TFL is taking the strain. That muscle, which runs from the pelvis down to the knee, is super-tight and seems to be pulling the knee slightly out of place. Hence the pain.

I have to teach my brain to use my gluteus medius again. How do I do that? A rigorous set of exercises, combined with plenty of squeezing of the lazy area to make sure it's actually working.

And it gets worse. For good measure, Bell adds that my gluteus maximus (another bum muscle) is a bit saggy. And my transverse abdominus (one of the muscles in the abdominal wall) is under-used. I'm a mess.