in shape  (fit/in good physical condition)

out of shape (unfit/in bad physical condition)

Eg 1:

A: I’m going to start going to the gym. I need to get in shape before my wedding.

B: Yes, I think I might join you!

Eg 2:

A: How was your holiday?

B: It was really good, but I ate and drank so much and didn’t exercise at all. I feel really out of shape.

A: You’ll get back in shape once you start running again.

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Are you in shape or out of shape?

What do you do to stay in shape?