Did it start with your walking plan, Skywalker ?

It would not be unusual to have muscular back pain after starting a new walking program if you had been fairly sedentary before that. That happened to me last year when I started walking and trying to lose weight. If that's the problem......If you can take/tolerate the usual interventions, an anti-inflammatory like ibuprofen or Naprosyn, could help, as well as using ice to decrease inflammation after your walks, and heat and gentle stretching before your walks.