The thing is, the fitness level OP is talkign about is not going to be compatible with the grown up playground sort of activities. You need to get jacked basically. You need to be lifting to failure 3x5ish hitting all muscle groups 2x a week with compound lifts. You should be struggling to go up stairs after working legs.

Working out is good, yes, but you need to actually work the body and make it suffer so that it may actually be damaged enough for high rates of muscle growth. So many people do not do enough because they don't like to push themselves, they take it easy and they end up wasting their time in the gym. Even more true as you get older and natural testosterone levels start declining and limiting your muscle growth.

This is why people end up taking spin classes when they can just do it at home on their own stationary bike. For some people, they need a jockey who will kick their ass along with the peer pressure of a room full of people dumping out sweat.