I took up riding fixed gears in my 4th decade. Run a front brake at least and use it. Start around 70 gear inches and go up or down depending on how you feel. Foot retention is very important for control because you can control speed w/ your legs. When you shoulder check, don't coast. When you come up to a red light, don't coast. When you try to get your feet in the pedal retention, don't coast.

Don't try to coast or you'll go over the bars. The physics of it is simple: your legs go from moving to not moving so all your momentum carries you forward as does the pedals which assist you over. Couple that w/ a panic grab at the brake to stop the bike even faster and there you go sailing gracefully over the bars.

@Beanz: a flip flop hub can be fixed fixed/fixed free/free free. You can put whatever combo of cogs on either side although ideally you want them to both fit w/ the chain you have and still stay in the track ends. I've never used the freewheel on my wheel, but it's sized for fewer gear inches in case I need a bailout gear.