I've done a couple of 200k brevets and a fleche on fixed gear bikes. I figure if you are used to riding fixed, and can manage the distance on a geared bike, you will probably be fine on a fixed gear bike.

I rode one of the 200ks on a bike geared nearly as high as yours is. Since then I have geared down to about 74". Lower lets you spin a bit more, but limits speed a bit on descents. Higher was fine, but I figure my knees may last longer at the lower gearing.

I prefer to have two brakes on a fixed gear bike. It feels a lot more stable controlling my speed on long descents with the rear brake or both than with just the front brake.