The distance isn't an issue. I've done a century with over 6000' of climbing on my fixed gear. There are people in the LD forum doing hilly 1200k brevets (The Gold Rush Randonee) on fixed gear bikes.

Singlespeed lets you concentrate on just turning the cranks. No worrying about shifting or anything. I do about 25% of my mileage on fixed or singlespeed, including a commute with a 2 mile long climb. If you're prepared for it with a decent gear ratio, a single/fixed is a great commuter.