But how much is the right amount?
There are all kinds of theories regarding how heavy you should lift, how many reps to build muscle, how many reps to tighten, and how intense you should make your workout. But whatever your goal or workout preference you should always struggle. That is, you should always be pushing yourself just a little further then you can go.
But that doesn't mean pain! That means, you don't stop when things get too heavy, you shoot to get out one or two more.
It means, you don't leave the class before it's done, you rest a minute if you have to and keep going.
It means you pick up a weight a little heavier than usual because you can now easily do the 10 or 15 reps.
Struggle is part of success, and not just in fitness (obviously). Struggle is the place of growth. Comfy means staying right where you are.
So next workout, be sure to push yourself to "struggle-mode" to make it into victory-mode".