Hi TF - that's pretty good for your first walk. I used to walk home from work and 4.5 miles used to take a little over 1 hour at a steady pace (and with regular practice!). My lower back used to ache too and if it's any consolation it will ease off when you get more used to it. I think the hot belt thingy would be good though - look forward to finding out more.

Keep up the good work