I do plenty of exercise activities in the game. You can do amateur boxing and then pro boxing that is a fun way to gain muscle.
After you complete the Family Trouble story you can exercise with Anna and Phoebe which can be a lot of fun.
Other than that cheats are your only option. Some players love the grind and some players hate it so cheats are available to use if desired. I'll repeat what noobtrain said, "You can customize the level of grind that you are comfortable with using cheats."
https://f95zone.to/threads/evil-2-v0-61-community-project.41582/post-8688661
How much grind is in the game is largely up to you!
If you only want to cheat a moderate amount, use this cheat:
dynamic $d_pimp_me
If you want to cheat a lot and gain the mansion, use this cheat:
dynamic $d_crushboss
Alternatively, if you just want to cheat muscle, use this cheat:
mus[you]+=1000
As you gain fitness and fitness skills, your Energymax stat will increase automatically. The max is 12.
Good luck!