歩く - to walk
元気 - genki, energized, healthy, feeling good, etc. (very important word)
疲れる - to exhaust/be exhausted (tired from work/activity)
腰 - lower back
痛い - hurt, in pain, painful
かもしれません - unsure, can't hazard a guess, probably, probably not...
When you walk, you recharge your batteries. But, when you walk too much, you get exhausted. (Plus,) your lower back might start to hurt.
腰 is only the lower back.
背 is the whole back.
背 is used only for height: 背が高い means "tall", not "your back is high."
Also, don't forget 肩, shoulder, and 背骨, backbone.