It is of course best to only bite off what you can chew. And many workaholics are accomplishing just as much as someone who puts in half the hours. But! If you find yourself in a long-term all-work-and-no-play situation, what physical and mental risks do you face and how can you best mitigate them?
I'm on my second large business start-up. When my partners and I on the verge of mental and physical breakdown, we say "holy karoshi!" making light of the Japanese term, "death from overwork." But sometimes it seriously feels like we risk that. Please give specific advice. Meditation, exercise, etc. are of course acceptable answers...hopefully, however, an empathic and experienced soul has some deep thought to offer here.
The first way to mitigate the risk of burnout from overwork is to, wait for it, NOT OVERWORK! :) Seriously, everyone has a different work/play balance. I find much of what I do, because I love it, part-work, part-play, so I can take on more "work" than most. Find what yours is. Can you work a 40hr, 60hr, 80hr, 100hr week? With that in mind, I set fixed blocks of time for work, and fixed blocks of time for play. And, there has to be a CRITICAL reason to deviate. Furthermore, if I must borrow time in one block, I make sure it gets moved to another area.
Secondarily to that, managing a hectic, entrepreneurial life is all about disciplined organization, after you've circumscribed your time. For that, I use a personal combination of ideas from Scrum, and from GTD, to make sure I know what I am on, what's coming and, most importantly, what I can commit to within my allotted block. That reduces the stress level immensely. IMO, stress, which leads to health and mental issues, comes more from seeing the looming pile of demands, feeling like you must commit to it yesterday, and then don't deliver.
I call it "Block And Tackle". It works very well for me.
Lastly, you have to differentiate from stress from spikes/events in the business, and from not managing your day-to-day properly. The former is acute and unpredictable; you only weather it by positive attitude. The latter is much more fixable and much more important to fix. As an analogy, the body can take one night without sleep, but can't take not-quite-enough sleep on an ongoing basis. Of course, a week of sleep deprivation will definitely kill you! :)
I would suggest looking at how you can restructure your business so that the work pressures are less, can you tell us more about your startup. the four hour work week has some good advice on this Doing things like