Just do it. I always take a bathing suit- never know when I'll find a pool. Also have workout clothes. Prefer a gym to running- I'm 51 and it's hard on the body. I gave in and went running in the Baltimore Inner Harbor area on a beautiful day last March and had shin splints for 3 days.

Also agree with walking whenever possible. I tend to carry a laptop plus a small tote bag on board and by the time I've wandered all over the airport I feel like I've hiked 10 miles.