I am 51.8 and like you fit. I started single-speeding a month or so ago, riding mostly with the freewheel (including 100 miles yesterday in the TA century). I've been riding all my life. I'd recommend a bicycle with both fixed and free, and two brakes. I start with the freewheel on the city streets and save the fixed gear for a ride in the park, preferably a flat one. It is not that I don't think you can do it, and it is not that it isn't great fun. It is exciting, challenging, fun. But it takes some getting used, and until you get used to it on city streets it is dangerous (especially but not only without brakes). It can also be very rough on SOME people's knees and backs. You may have noticed that the vast majority of fixed gear urban riders are young. There are several reasons for that, all simple. Young people consider themselves immune from mortality. I know I did when I was young. Even when faced with it--a near accident or even an accident--they bounce right back. Second, their bones are much soften, much less brittle. Even though they are wrong to think that they will never die, they are right to think that they can take some pretty nasty falls and get right back up on the bike. You and I are much more susceptible to broken bones or strains and sprains that take months and months to heal. My worst fear is not being able to exercise, so I make some compromises to ensure that heavens willing every single day I can, including today, after 100 miles yesterday. The final reason: kids are nuts. (Just kidding). Go get a bike and enjoy it, and if you have additional questions you are embarrassed to ask the kids, feel free to ask me. I bought a Sputnik, and I am having the time of my life. I am unlikely ever to buy a bike with gears again.