Not only should the time be regular, the amount of exercise also should
be about the same each day. We should not give a half hour or an hour
one day and neglect it entirely the next any more than we should eat one
extraordinary meal and then go without anything to eat for two or three
days.
The same is true also regarding the kind of exercise. It may be helpful
to change some of the exercises, but we should have exercises for all
parts of the body. If we substitute one exercise for another we should
take care to exercise all the parts equally. We may change the kind of
food, but the degree of sustenance it contains should not greatly vary.