Beware when you hear:
1. I run + walk a lot that counts as leg day.
2. I just want a nice butt & don't care about the rest...
A well-structured leg day is a lot more than just moving your lower body extremities until you break a sweat.
1. Focus on larger muscle groups, like glutes and lower back, quadriceps and hamstrings.
2. Then fatigue the smaller muscles groups in isolated exercises. It's ok to break lower body muscles into two groups and hit them on different days if you train 4+ days a week... however if you're a newbie, or lift 2-3 days a week, gotta address all leg muscles