Stretch to increase flexibility. I am 55 and am a martial artist. I cannot impress on you enough the need for staying limber as you age.
Shooting prone requires use of different muscles than you are used to using. Train by getting into position and keeping your head erect for long periods. This will strengthen your neck and improve your shooting.
If you are becoming leery of scope bite, either get a scope with longer eye relief or shoot milder cartridges.