I don't think I could go to a gym and exercise just for the sake of exercising and I'm guessing a lot of internally-motivated startup guys/gals are the same way, so I encourage you to pick up some sort of exercise that is also FUN.
I wake up at 5:00 am on Tuesdays and Fridays and go bouldering at Planet Granite (In Belmont, I'm currently located in Palo Alto (well, for a short time anyways))
For rock climbing/bouldering you need a fair bit of effort and it is also a very enjoyable exercise (it's kind of like playing).
This also fits nicely into the rest of my schedule.
I wake up at 5:00 am on Tuesdays and Fridays and go bouldering at Planet Granite (In Belmont, I'm currently located in Palo Alto (well, for a short time anyways))
For rock climbing/bouldering you need a fair bit of effort and it is also a very enjoyable exercise (it's kind of like playing).
This also fits nicely into the rest of my schedule.
Highly recommended