"What do you do to stay in shape?"

Staying in physical shape also helps the brain function better. It really doesn't matter if you are completely in shape or not. But being active indicates that you are not a lazy couch potato.

"I like to take long walks with my dog on a regular basis, and I also go on weekly hikes at a trail near my house."

"I work out at a gym a couple times a week. I like to stay active."

"I play for a co-ed softball team. It's fun and it gives me a little exercise."

All these examples will work. It doesn't have to be long but you should answer with something that indicates that you are active.