I've had much more time to work out on our 24hr. + International layovers and have lost weight since flying Int. But, you do have to say "No thanks, just a cup of ice please" to at least two or three inflight meals each leg, or you will end up looking like an International Flight Attendant. And with more days off at home, I get to spend a lot more time riding my road bike and going to my local gym too.
BUT as others have pointed out, you do have to come up with a way to get some sleep, both in the airplane and on the layovers, or you will be a mess. I won't drink coffee in the airplane until the last 3 hours, if that's my shift in the cockpit, otherwise I won't be able to sleep on my breaks.