I've done a few centuries on this old Claud Butler road-path bike (basically a track bike with road geometry).
It's a fixed gear about 72 gear inches. Riding east and south from my my central NJ home, down into the rolling hills and flatlands of New Jersey, that gear was fine. So a year ago I tried doing a hilly century, riding north and west from home... and that was brutal. I ended up walking up some of the hills. Getting up the hills seems to be a matter of technique; for best results, take it easy on the way down, save your energy for the climb, and get up the hill as fast as you can. If your cadence drops below a certain point, you're going to be walking.
I summer on Long Island, and a few years ago I set up my Long Island bike as a three speed fixed gear, with the Sturmey Archer S3X hub:
.
But I found the three speed unnecessary. It's now a single speed fixie; I really don't need more gears for those Long Island hills. I use this bike for all my recreational riding on Long Island, so five or six centuries each year.