Overtraining isn't the same as a specific muscle strain, but yes, working out 7 days per week is definitely overdoing it. Your body needs one full day of rest per week to recover and rebuild and get stronger for the next round. Here's a good article on the subject?
Was your diagnoses, and the stretching exercises, done by a doctor? With an injury, please don't dork around with advice unless it comes from a doctor or physical therapist.
As for prevention, sometimes leg/back issues are due to not having the correct shoes. This wouldn't be so much of an issue for Curves, but for your walking, absolutely. Most people, especially women, don't have perfect alignment & gait, so workout shoes that don't offer the correct support can cause problems when activity levels increase. You might visit a real athletic shoe store in your area (not some chain in the mall) and get fitted for proper shoes.