From September 2010 to March 2017, I did all of my road/distance riding on a fixed gear bicycle. I had a geared bike for winter/utility riding, but my main commuting and road cycling miles were on a heavy, steel fixed gear.
Going back to gears kind of broke me - I started overusing the gears, thinking I needed tiny gears for climbing and huge gears for descending (though I actually do need the latter). After ~9,000 geared miles, I've learned that grinding up hills a bit is okay, and tucking to descend can do as much as spinning out a large gear.
I also wimped out and un-fixed my single speed. After descending with a freewheel, I decided that trying to spin at 30MPH really is not that much fun. I may go fixed again once I move to an area without long/steep hills.