This can be done in any mode, as soon as you enable Fitness mode in the Fitness section. This is the first option on the list when you enter Fitness mode. As soon as you enable this, it will be saved as being on for your profile, and any songs you play in any mode will add to your total hours. This achievement will unlock naturally when doing all of the Crew Challenges and songs in Perform It mode, so long as you turn it on right away. The time counter DOES NOT increase when you sit idle; you must actually be playing a song. To check your progress, go into Fitness mode and look at your overall stats.