>The simple truth is that any (non-crackpot) regimented training program will significantly improve a geek's fitness.

Taking a daily walk outside is a good start. By doing so you shift your attention into a different environment and into moving the body. This somehow recharges the mental batteries, which in turn improves one's physical health.

One tweak can significantly increase the benefit at little extra cost. During the walk, take one short sprint (long enough to lose your breath slightly; it'll probably take less than a minute).

I can't explain why this helps but it does seem to help me. Something to do with keeping aerobic capacity in trim?