Along with all other sentient beings, I especially hate models at this time of year, when they drape their freakish frames across magazine pages in order to illustrate the lies about whatever pre-Christmas diets and exercise programmes the wretched publication has chosen to herald the festive season.

But it's always been a constrained sort of hatred, limited to these unknown, two-dimensional beings and mitigated by the knowledge that it is as reasonable to loathe winners of the genetic lottery as it is winners of the ordinary kind.

Alas, these constraints are no more. The mental boundary was breached last week as I was lurching away on the crosstrainer, lamenting the fact that my efforts have not yet yielded a single positive result. At this point the rear view of the woman on the treadmill in front swam into focus. She was running, effortlessly, on a gradient of about 300%, unsweaty ponytail swinging, skin glowing, lightly muscled arms pumping, her bum looking like two snooker balls in black lycra. If I'd got close enough I'd probably have discovered she had enough breath left to sing the Oklahoma! score to keep her spirits up. She can do that, I thought, and I can't even go on the frigging treadmill because my belly wobbles so much that it makes me cry. And something snapped inside me.

Since then, my odium has not been bounded by rational thought. I've been walking the streets in a bile-filled haze, so liver-witheringly jealous of the swathes of the population who are slimmer or fitter-looking than me that I periodically become weak and have to prop myself up against a lamppost until the fit passes. I want to wrestle to the ground every one of the gazelle-thighed millions who walk past and ask them, "Are you entirely heedless of your good fortune, or do you have the decency to wake every morning and spend 10 minutes on your knees, thanking your unnamed but glorious god for the bounty he has given you?" If their answer is the former, they go straight under a bus. Filled with fat. And driven by me.

· Next week: Tim Dowling does yoga.