Maintenance level exercise, when necessary, has a very low required amount.
Under 5 minutes of calisthenics a day can, and does act sufficiently to keep you from ever regressing.
You simply need to hit a little for most or all of the body.
•25 pushups
•10-15 bodyweight squats
•3 pullups
•Flexing
That’s guidelines, it’s not set in stone. Divide the body into push/pull/legs, and just get it in.
5 minutes or less, able to be done everywhere, and you’ll never backtrack on ability.
Fitness is easily maintained when you keep a PT mindset.