Here's what I did to incorporate physical exercise into my routine:
I went to a cleaning contractor and got a part-time job (16 hours/week) cleaning apartment buildings and offices that require weekly cleanings. I arranged all these jobs to fall on Mondays or Thursdays. In exchange, I work for my startup on weekends as well, so I still get a full-time job worth of time for the startup. I also picked only the jobs within 4 miles from my house, and convinced myself that I'm too cheap to pay for a bus or to buy a car (sadly, that wasn't too hard :-).
Here's what I get from this arrangement:
1. Steady physical workout (cleaning + walking there and back) that is perfectly sufficient for general health (not for running a marathon, but I could run from work instead of walking - that might do it). I lost 20 pounds after 10 months of doing this without changing eating habits or anything, I sleep much better, and my heart palpitations are gone.
2. I cannot skip this workout at will, because I'm obligated to do the work, or have a very good reason why I can't (or get fired and pay a pretty big fine). So I don't have to force myself at all - the external pressure is sufficient, and I can save all my willpower for the startup (and I do need every ounce of it).
3. Mental health: doing a MEANINGFUL physical work is very fulfilling. You can actually see the results of your work, you can see the world around you changing for the better, becoming cleaner. That's very rewarding, especially for programmers, whose contribution is virtual, not physical, especially in the early stages of the startup when it often feels like you're doing something meaningless, and you have to fight this feeling constantly. A physical, results-oriented, community-beneficial activity is a nice, perhaps even vital break from a startup. Whatever the reasoning, I do feel it - the simple mindless work clears up my head better than anything I've ever tried before (though I've never tried drugs :-).
4. Time well spent: to my own surprise, spending two days per week cleaning doesn't waste time. Since the work doesn't require much brain power, I can do it automatically, while thinking on the startup and other things. I got so many cool ideas while cleaning, that at some point I downloaded a memo app so I could write these ideas down as they come (and they do).
5. Finally, I get paid for this job just enough to cover the apartment rent, which alleviates the financial pressure of an early-stage startup. I think of it as a gym that pays me for my workouts! How cool is that?
Of course, cleaning is not the only option with all the above benefits. There's also construction work, gardening, dog-walking, non-express delivery services, shopping for disabled or old people... you name it. For some of these you can even get community service points.
I think every startup founder or self-employed person should try this method.
If I had time, I'd create a startup around this idea :-)