In reply to jules699:
I'm currently riding a fixie around the countryside, and it's good for your fitness. The key thing is to get the gear inches right - google sheldon brown gear ratios - 'gear inches' is the distance you travel for every revolution of the pedals. I've currently got a 70 inch gear (48:18) and that's OK. Good eough to get me up Box Hill in Surrey and over the Malvern Hills, but it's hard work for me - I'm just about totally trashed at the top, but the thing about fixies is that you've got to keep going, so no coasting at the top.
We've recently put together another lovely fixie at home - light as anything and a former track bike with new forks. It still has its track bike gears though - and 49:16 is just too much (that about 82 inches, or another foot for every revolution). At that rate, you feel every little incline, but are really quick on the flat. It makes for some impressive overtaking manoeuvres, especially as the passee doesn't know you're only just managing to hold on to your breakfast!
You'll be fine. I'm in training for a 100km ride around the Surrey Hills - I might bottle it though and borrow a bike with gears - it finishes with a timed lap of Box Hill. 'm somewhat chastened by the LETJOG on a fixie aove though - i might have to man up a bit