While on the go, it's hard to maintain the same routines you have back at home. Similarly, you're unlikely to find a gym while you travel in some more out of the way places, and you're sick, stuck on a bus, etc etc. You're eating strange foods and drink, and often partying.

Aside from tongue-in-cheek backpacker workouts, how can you keep fit / work-out while doing long-term travel?

I personally find I lose weight when on big trips, because I have a sedentary lifestyle when working, but that's just weight loss, not working out, per se.