The high profile ones seem to either go into seniors touring, or become coaches, so both of those will keep them fairly active.
It also seems that the top players don't just eat what they want, expecting to burn it off. They have specialised diets and treat food very much as a fuel. Looking at that list of what Michael Phelps eats, it seems like a chore to get through that many calories. Eating a regular diet may feel luxurious by comparison.