My wife runs marathons, she is in a running club, and yet I notice the more she runs, the more her body adapts for her to get the same out of it.
So for example if she ran a mile, for two weeks, she would then have to start running 2 miles to get the same sort of benefit out of it, as her body adapts and manages this.
Part of the training is running in stops and starts up hills, 30 seconds sprint, 15 seconds jog, 30 seconds sprint.
Yesterday I did a small experiment..
I live on a hill, at 13.50 I did my blood, 7.4 then walked down the hill, and back up again at a fair pace, I got back 13 minutes later, puffing like a train..(over weight, under exercised).. I waited to recover, it took 3 minutes, and then re did my blood..6 it stayed there all day until tea time, when it was 5.7..I din't think you have to walk for miles, to get benefit from exercise, I think it's the effort rather than the duration that counts.
Now don't think for a minute i'm saying you shouldn't do whatever, I would never do that, or convince my other half from running bonkers miles, however, I think for people who think exercise is to much of a drag, takes up to much time, etc. for me it seems a quick 20 mins quick march up a hill, gives me the benefits of a leisurely 5k jog