To do this, you can either buy all 3 Fitness books (which'll rank you up to level 9) and/or buy gym equipment ($160-$3,250) and keep practicing on them. You can also go for jogs too to improve the skill.
If you use the 3 books, you can only level up to level 9, so you will have to choose one of the other options too.
Apparently if you load up household Caliente it'll pop straight away, I haven't tried this but if you would like to then be my guest.