i am in the process of building up an old mountain bike as a singlespeed rather than fixed, but i was thinking of buying a flip flop hub to be able to do both. Fixed wheel offroad would be, well, entertaining!
Way more popular in the last few years, and definately improves fitness if you ride already
But locality matters for that, i will only use it in the woods/ forests. Most of my riding in in the hills, i stay on the edge of the grampians, so you really need two gears one small one large, a medium just doesn't cut it! Might get away with it on a road bike, which i am guessing it your arena?