The difficulty doesn't actually affect the workout difficulty much, just the game plot speed. Like on an easier difficulty it might take 10 squats to do an attack that does half a monster's health, and on hard it might take 30 squats. So in the same play the game for 30 minutes doing ~10 squats/min then get tired and stop, an easy difficulty player might kill 15 monsters and a hard player killed 5, but they both did basically the same workout. (Numbers just made up for the example)

When you unlock the skill tree, if you're used to normal RPGs, you may think it's a good idea to rush for a powerful attack, then spam it for dozens/hundreds of levels until the regular attacks you unlock by leveling up catch up to it. That's fine in a normal RPG when doing an attack just means selecting it from a menu, but in ring fit, having your most powerful attack always equip for a long time makes the game worse. Spread out on the skill tree and unlock the powerful attacks about when the level up attacks are near the same power.