"By the time I've got changed"

Of the five reasons in chapter 2, the fourth was the overhead. A gym session has fixed costs. If travel, changing and showering take forty minutes between them, then fifteen minutes of exercise is a poor return on fifty-five minutes of your evening. I said that reasoning was sound as far as it went, and it is. It goes wrong in assuming that exercise means the gym.

The overhead belongs to the full session and not to exercise as such. Take it away and see what's left.

A brisk walk needs no kit. You can do it in work clothes and ordinary shoes. You go out of the front door, walk fast for five minutes, turn round and walk back. There's no travel, because you start where you are. Unless it's a hot day you won't need a shower, because a moderate pace doesn't soak you. The NHS gives a simple test for moderate intensity, which is that you can still talk but you couldn't sing. It should raise your heart rate, make you breathe faster and make you feel warmer. A properly brisk walk does that for most people.

Stairs need no kit either. If you live or work somewhere with a staircase, several trips up and down at a good pace will have you breathing hard in under five minutes. The Jenkins experiment in chapter 4 was nothing more than this.

Ten minutes of movements on the living-room floor needs a space the size of a rug. Squats, press-ups against the kitchen counter, stepping up on to the bottom stair, marching on the spot with high knees. You'd want to be out of your work trousers for that, perhaps. It's still two minutes of overhead and not forty.

Once the overhead is gone, a second fact comes into view, and I think it's the most practical point in the book. You have few ninety-minute blocks in a week. Most working adults with any family responsibilities could count them on one hand, and those blocks are fought over by everything else you want to do. But you have a great many ten-minute slots. There's one while the dinner's in the oven, and one after you've dropped someone off and before you need to be anywhere. The second half of a lunch break is another. So is the gap between the end of work and the start of the evening, the very gap you stood in on Tuesday and called useless.

If exercise can only happen in ninety-minute blocks, it has to compete for the scarcest thing in your week, and it will often lose. If it can happen in ten-minute slots, it's drawing on something you've got plenty of. This is my own reasoning and I know of no study that counts people's free slots. But I'd invite you to count your own tomorrow. Note each time you find yourself with ten minutes that aren't committed to anything, including the ones you spend on your phone. I'd be surprised if you found fewer than four.

An objection. "I don't want to be the person power-walking round the block in office shoes. I'd feel ridiculous." I understand that. Two things may help. The first is that nobody can tell a person walking briskly for exercise from a person who's late for something. The second is that the feeling of ridiculousness is the threshold talking again. It's saying that this isn't real exercise and you'd look like someone pretending. Chapters 3 and 4 were about whether it's real, and by the evidence it is.

A second objection. "I've paid for the gym. I should use it." If you like the gym, use it, on the days when the block is there. This book never asks you to give it up. But the thirty-four pounds is spent whether you go or not, and it shouldn't be allowed to define what counts. On a day when the gym can't happen, it's no use to you, and the walk round the block is worth the most valuable fifteen minutes on the scale.