FWIW: Obviously, I am no bike athlete but at age 70, I started doing 45 minute workouts with serious weights. Today at age 80 my bone density is excellent and I am in better muscular-skeletal condition than at any time since age 40 or 50.  Biking three times a week (I ride three recumbents, a Lightning P-38, a Volae Team and a Nautilus exercise bike) and doing weights three times a week make for excellent cross-training.

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