60+ minutes of exercise per day to maintain weight after age 50

[malkin]

It was a suggestion in Runner's World, the idea being that if you go out for a mile, you're likely to go further, and even if you don't, it's still a mile.

In for a dime, in for a dollar.
I do the same with practicing piano. If I sit down at all, I know I will do more than just a little.
(Of course it usually works the same way with drinking beer too.)