The decision
Here's the rule I'm suggesting.
I get up every half hour, and I walk for ten minutes every day.
It has six parts, and each one is there for a reason given earlier in the book.
Every half hour means every half hour of sitting. When you've been in a chair for thirty minutes, you stand up and move about for a minute or two. Walk to another room, climb a flight of stairs, fetch something, or just walk to the window and back. Then sit down again if you want to. I've chosen the half hour myself. Dunstan's experiment used twenty minutes, the NHS gives no figure, and nobody knows the ideal interval. Thirty minutes is frequent enough that a long sit can't build up, and it's easy to keep track of.
Use a timer for the first fortnight. You don't notice time passing in a chair, as the tally in chapter 1 showed. A kitchen timer, a phone alarm or a watch reminder will do it. Restart it each time you sit. After a couple of weeks most people find they've developed a rough internal sense of it, and the timer can go back in the drawer.
The break doesn't have to be exercise. It only needs to be upright and moving. Light movement did as well as brisk in Dunstan's laboratory. If a break happens to include the stairs taken fast, that's better still, and it's the kind of burst Stamatakis measured, but it isn't required.
The ten-minute walk is separate, and it's brisk. Brisk means you feel warmer and breathe a little faster, and you could talk but not sing, which is the NHS test for moderate activity. Do it whenever suits you. After lunch and after dinner are both good times, for the reasons in chapters 4 and 11. Ten minutes is a floor and you're welcome to go over it. If ten minutes of brisk walking is beyond you at the moment, walk at the pace you can manage and let it build.
Evenings and weekends count. The rule applies on the sofa as much as at the desk. Chapter 11 argued that the evening is where it matters most.
A missed break is just a missed break. You'll look up and find you've been sitting for an hour and a half. When that happens, stand up. Don't do extra to make up for it and don't write the day off. The rule is about what you do next.
Plans of the "when this happens, then I'll do that" kind make a decision much more likely to survive a busy day. Peter Gollwitzer and Paschal Sheeran pooled 94 tests of such plans and found that they made people a good deal more likely to follow through. Here are three, and you can change the wording to suit yourself.
When the timer goes and I'm in the middle of something, then I'll finish the sentence I'm on, stand up, and come back to it in two minutes.
When I put my knife and fork down after dinner, then I'll put my shoes on before I do anything else.
When an episode ends, then I'll stand up before the next one starts.
Tell the people you live and work with. It saves explaining yourself twenty times, and one or two of them may join you.
Once the rule has settled in, which I'd give a month, there's a natural next step, and I'd be leaving something out if I didn't mention it. The NHS asks adults of every age to do activities that strengthen the major muscles on at least two days a week. For the purposes of this book that matters because strength is what decides the stairs, the floor and the shopping, and it's what Fiatarone's residents rebuilt. It doesn't need a gym. The NHS website has strength routines that use your own body weight, and its list of strengthening activities includes carrying heavy shopping and heavy gardening. If you haven't done anything of the kind for years, this is the point at which the advice in the preface about talking to a GP applies most.
What to expect from the rule itself I've covered in chapter 13. The first few days may bring aches in your calves and hips, which is what happens when muscles that haven't been asked for much are asked for a little more. It should be mild and gone within the week. Pain that's sharp or that persists, or any of the warning signs in the preface, is a reason to stop and get advice.