It depends where I've been the night before. I'm not very sentient when I look in the mirror in the morning; it's a blur, which may be for the best.

When I'm writing, my standards slip. I wear the same clothes for days and don't shave. If I see myself in the mirror, I think, "Oh my God, you're a mess." The plus side is that when I write, I don't go out carousing – I don't do drinking and late nights, because the recovery time takes too long. You can waste a day of writing. So in some ways I can scrub up a lot better.

I do notice the ravages of time on my face: I see the laughter lines, the frown lines, battle scars. I can get obsessed by my gut. I'm always scanning it – is it getting bigger, does it look extended if I drink? Sometimes I think it would be great to get some liposuction and get this bastard taken out, but I think I've accepted the six-pack is beyond me now.

I do look after myself. I run two or three times a week and go to the gym. In some ways I'm in better shape than I was 15 years ago – I'm 50 now. My wife is 28. If I hadn't been with someone a lot younger than me, I may have let myself go.

Anybody who doesn't have a fear of age and mortality is a liar or in denial. But you have to be graceful about it and accept you can't be the same as you were. There's a misconception that when you look well or fit, you can look younger. You can look good only for your age; you can never look how you did at 30.

• Reheated Cabbage by Irvine Welsh is published by Jonathan Cape.