I think the most important thing you can do to make exercise a persistent part of your life is to make it a social activity. At this point, I don't run to stay fit. I do it because it's a great way to hang out with my friends while doing something fun. Physical activity doesn't have to be medicalized or mandatory -- it's actually much more enjoyable as an activity you share with others. Going for a comfortable run with a few friends or going rock climbing with a buddy is addictively fun -- I just wish I had the time to meet up for it daily instead of only 1-3 times per week.
If you're in Boston and thinking about starting to run, I've had a great time running with The Most Informal Running Club Ever on Saturday mornings -- http://informalrunning.com . Nothing beats getting together with 30-60 new and old friends, running by the Charles at a distance and pace of your choosing, and then hanging out with them all for a potluck breakfast.
Our culture for some reason thinks exercise is prescription medicine that should be taken in secret. It turns out that exercise is just like food -- best enjoyed in a convivial group.