It really comes down to finding something you enjoy doing, so that you'll continue to do it. Also schedule time to do it.
I hate gyms, so I like riding my bike. I ride to work a few days a week (17 mi each way!). But if you can't do that and don't like the gym, maybe look for rec league sports such as b-ball, volleyball, tennis, racquetball etc. That way you'll be scheduled and somewhat obligated to show up. Getting started is always the hardest part. It's all about inertia... when you're at rest, you want to stay at rest, but once you get moving it's easy to keep moving. And once you're done you feel good both mentally and physically (and all the other crap about sleeping better and needing less of it is another benefit).
If nothing catches your attention, try going to the gym (schedule time for it) and look for classes they offer. They offer things like spin, aerobics, kick boxing, and other stuff. Maybe something will interest you.
I'd also bet that if you found an activity such as tennis or racquetball to be interesting, but didn't have anyone to play with, you could find web forums for it and find locals to play with. I know that if I ever want to go mountain biking with someone, I can go to MTBR, and it wouldn't be hard to find someone to ride with (although I enjoy riding alone and with a few specific other people).
Also, you got kids? There's your motivation! If you don't do it, you're cheating them out of a parent when you die younger than you should.