Geared Bike so Much Harder Than Fixed?

Stormy 2 weeks on and I'm giving up gears, I did a ride today of 16 miles and felt knackered after a mile and my legs were killing me by half way, wasn't even hilly or windy, my legs have never felt so tired as they do right now, previous 9 months on fixed and they never felt tired

back on fixed from now on

Do you have your brakes on all the time or something?