During the day, go to the Potion Shop and buy a Stamina Potion. (You can actually buy both of them, but, either way, you’ll have to wait a while to finish the quest.)
Sleep until nighttime in the Knight Academy (preferably). At the Knight Academy, go into Fledge’s room (next to Link’s own room) and speak with Fledge, who seems to be starting a workout routine.
However, he lacks the energy to keep it up and will probably quit. When he notices, give him a Stamina Potion. Do so again (later on, if you’re broke) on another night.
Once you meet the reward prerequisite above, visit Fledge to obtain your five Gratitude Crystals.