Wow these are all great responses. I am surprised by the number of mountain biking recommendations...might have to give it a try. My body is beginning to tell me that other, high-impact sports, which I have always enjoyed, may not be an option for much longer.
I think one big thing that has not been addressed is that when you hit your late thirties you start running out of things to look forward to. This forces you to take stock of your life. You look around and realize that your are living in a reality that you have created for yourself. Whether you like it or not, this is it. This is your life. This is what you've been working toward for thirty some-odd years. It's a bit of a point of no return I guess like you are steering a huge ship that has picked up so much momentum it is next to impossible to turn around for anything you've forgotten.
As a kid there's always a milestone ahead in the near future...becoming a teenager, new school, first girlfriend/boyfriend, driving, college etc. Then adulthood hits and everything is new and fun as you start a new career. Then comes marriage, new house/apartment, then kids...then WHAM! Brick wall. Life all the sudden plateaus. The more responsibilities you have collected for yourself through young adulthood, the more trapped you feel. "Responsible" people have lots of "responsibilities" and probably feel this pressure the most.
For me, I woke up one day and realized that this classic family life in the suburbs that I've spent my whole life working toward is just not enough for me and I have no idea what to do about that. I just know that I am desperate for a change from my daily routine. So much so that at times I feel like I am going to explode. I can see where some people would decide to just blow the whole thing up and start anew but this will not be my path. I know my family will be a part of whatever I do going forward. I also know that what I'm likely to do, going forward, is absolutely nothing...just put my head down and wait for the next phase of life. This feeds my frustration. I'm sure this is how most people feel to some extent.
Thank god I started saving early and am, at most, 10 years away from FI. In the mean time, I have started taking classes just for fun. Any time I see a course that interests me, I sign up. I am seeking more outlets where I can take time for myself.
Thanks for the suggestions so far.