Fitness is important - be weak and your game will tail off towards the end of the round.
You need variety as a young man to stay fit - Play Football / Tennis / Bowls / Cricket / Running / Swimming and finally Gymwork. Make it fun - surround yourself with friends that you can have fun with.
As you get older your community shrinks - you find yourself having to look after yourself. Swimming is good the gym is good and Squash is good too - take spinning classes and even try yoga to keep you subtle
As you get into old age and the body is no longer your own try to persuade your golf club to let you play Twilight Golf - Say 18 holes over the course of a fortnight. Try to walk the course with one club to see that you can still hit the ball.Sadly you will find that your value as a member is not in the interest of the club to keep you playing - It amazes me that clubs do not have Twilight Golf for the disabled of members with young families.