24 Hour Fitness promotes a variety of these deals every year. I signed up about 18 months ago for a similar deal ($607 pre-pay for three years, $99 each year after that--cancel anytime). My deal includs all clubs, nationwide. I think it's a pretty sweet deal, even considering putting up with the lousy music and too many machines crammed into the building.
My understanding is that if you move to a city that doesn't have a club within a certain number of miles of where you live, you can cancel your membership. Sounds like you got decent deal, but could have done better. The best deal I've heard of in my area (Portland, OR) was $525 for three years, and $20 annual renewals after that.
Yes, there is a risk when paying up front but I'm confident that this company isn't going out of business anytime soon.
Now, get out there and lift!