1.) In general, programming is a well paid, lucrative profession. It takes a long time to learn how to program well, so naturally the rewards are large. However, these rewards really don't mean all that much if you aren't in a position to enjoy them. Becoming a millionaire before you are 40 is cool and all, but most people would rather have a long lifespan full of good health.
2.) Programming naturally leads to a large amount of time sitting down in front of a screen. It is also a pretty high stress job, which often results in excessive use of caffeine or other stimulants. Both of these habits are draining and unhealthy. In the long run they are bad news.
My biggest challenge when working out is the mental focus required. It takes a lot of energy to manage your sleep and eating schedules to the necessary degree. Also, there is a lot of background knowledge and thinking involved in planning proper work out plans. Bottom line is that as a Computer Science student, I work on things that require a large amount of mental focus every day. Adding another item to the list is of course difficult.
Having been in excellent shape only a year or two ago (I was a competitive swimmer in high school), I can say with full confidence that being in shape has a huge impact on how you run the rest of your life. I found that I was able to accomplish much more with my time and take on larger loads because of the discipline I learned through being an athlete. There's no question in my mind that staying in shape is worth it, but the difficulty in doing so should not be ignored. I am still struggling to adjust to the demands of a Computer Science education. In high school swimming competitively pretty much meant working out 4 to 5 hours a day all total. That's simply impossible in college when studying any type of Engineering or Science. I'm sure it's also impossible for a founder/working professional as well. Instead I'm trying to figure out how much I can accomplish with only an hour a day. With proper dieting habits, I'm convinced the hour will be enough, but it is a challenge. It will be worth it, but it's not easy!
Friends I've talked to who also struggle with these types of issues talk about how they don't have time. Obviously I understand that, and I'm sure most readers of news.YC can relate :-). Ultimately though one hour a day is a small price to pay. Mentally programmers are capable of doing some pretty crazy things. Anyone who's done an all night coding marathon to finish a project on time knows what I'm talking about. Getting in shape is simply a matter of redirecting that mental energy towards a different cause.
EDIT: Something I forgot to add. Another big barrier that people face is an aversion to going to the gym, where there are people who are in really good shape. It certainly can be intimidating, but have faith in the fact that your body is biologically designed to respond to regular and difficult work outs. The difference between you and the guy next to you bench pressing twice your body weight is a lot smaller than you think.