Context

If you have a 1-hr run today and a 1-hr run tomorrow, then that’s what you should do. If you have to cut short your run today, say to 40 minutes, you should not add 20 minutes to tomorrow’s run under the expectation that 40 + 80 minutes over two days equals 60 + 60 minutes over two days.

Here’s why.

In the first 40 minutes of your run today, which turned out to be the total run, you got the vast majority of the benefit of that run. What you would have gotten in the next 20 minutes is a tiny bit of additional stimulus for additional adaptations.

If you add 20 minutes onto your run tomorrow, you’re running for 20 minutes AFTER already running for an hour. That’s 20 minutes under whatever conditions and state your physiology is in after the hour of running, which is different from the state your body was in after 40 minutes of running the day before. It’s a different 20 minutes and it’s a different stimulus. Exactly how it’s different depends on many factors, like the glycogen state of your muscles, your body temperature, your pace, etc. but the point is that it is different.

The next time you're tempted to ‘make up’ for a run cut short, be sure that you see what you’re doing clearly. Are you changing the stimulus of the extended session so that it no longer serves the intended purpose (or serves it less well)? I’m not saying that it’s never useful to squeeze that missed 20 minutes into the rest of the week somewhere. Just be sure you appreciate those minutes will be a different stimulus when they are in a different workout. The context of those 20 minutes matters.