I think it is even easier to stay in shape in aviation then it is out of it. I typically have around 14 hours give or take on a layover and that leaves me plenty of time to get a run in. I typically scout out some great running (walking if you dont run) spots at each overnight and it gives me a great oppurtunity to explore the area. If you do not want to run or the weather will not allow it there is always a gym at your disposal.
Its hard to drag myself out on the streets after a long day but no one ever looked back on their day and said "I sure wish I didnt work out today."