"maintaining a large amount of lean muscle is essential" I doubt about this. Maintaining exercise sure, maintaining low fat tissue sure, mintaining essential muscles comes as a result of daily activity: walking, using less devices and more manual processes, taking stairs rather than elevators, this makes more sense than going to gym, because diverse muscles work, in a more natural and physiological way
Look at centarians profiles, they're light, active, and not eating a lot (very little meat, mostly plant based)
