During the week I do ~120 miles by commuting to work. On the weekends I'll ride between 50 and 250 miles, depending on if there's an event that weekend.
I don't ride solely singlespeed, but about 25% of my mileage is on my single and I rode it fixed on a century last year with around 6000' of elevation. I didn't eat any differently than when I'm on my geared bike, and my total time was around 8h 15m.
__________________
"I feel like my world was classier before I found cyclocross."
- Mandi M.