Well, the common wisdom goes, you never know unless you try. I think that applies here.

From a guys perspective, don't worry about your age or being "supper fit". Sure, do your normal routine to look good but don't stress over the little stuff. Most guys, at least any that you would want to be in a relationship with, will have realized a long time ago that your personality and intellect matter much more than weather or not you look like your 20. (In my personal opinion, I tend to prefer women who don't look like they are still half way stuck in their teenage years, either physically or mentally.)

Anyway, just put up your info on a site and see what happens. You might meet someone you really like, if you don't than there is nothing saying that you have to meet them or even talk with them further. Take the chance and see how it goes. It can't possibly be worse than what your dealing with now (loneliness) and has the potential to be very positive for you.

Give it a shot, good luck and I hope it goes well.