Tips? Yes... Train! Hahaha
Strength training in the weight room, flexibility training in the yoga studio and boxing training with Uma ALL count toward building MC's physical fitness. So, train... In the early game, I went to the gym every morning and school every afternoon until physical and mental skills were maxed. Then I worked on PC skill every evening. Once PC skills are maxed MC can work like 2 hours each day to rake in a big pile of cash. I highly recommend doing that too.
To view MC's current stats click on the backpack, then select the icon along the bottom for player stats. That will allow you to track your progress with MC's fitness, PC skills, LI interest status and other stats. But, train or you can install the cheat mod linked below where you DL'd the game on the main page of the OP.
It's been a really long time so I don't remember the exact total needed but once MC is ready, the Xvideoz quest stages automatically trigger when you enter the weight room. Otherwise, just keep on training. Player's max fitness score is 10, just like PC skill and other skill stats.
Hope that helps you, friend. Adventure ever on, Phat