+1

Targeting muscle groups was VERY abstract at first (at least to me)

It is something I learned when I started lifting weights. I could never build my chest because I had very strong shoulders, so it really is best to learn early on so you don't get extremely dominant muscles, which will make it much harder. Really try to become attuned to what muscles are "firing" and from there try to shift how you generate the energy you are transferring into the pedals. This is one of the somewhat cerebral part of sports that can be a huge advantage, because in my experience I have found that rotating muscles groups because of fatigue isn't quite instinctive or natural.