Going slow is definitely a good thing while you're doing c25k. Slowing your pace a little will likely give your muscles a bit of a break. Repeating weeks is also a great idea. If you have a heart rate monitor, you might look at using that to guide your pace as well.

Shortly after completing c25k, I ran my first 5k. I completed it in 31:23 (10:07 min/mile pace). I figured that since I could run 10:07 pace, I should do all of my runs at 10:07 pace. A week later, I was looking at all sorts of aches, pains, and injury. I spent the next two or three weeks running with a heart rate monitor, using that to establish how fast my body would let me go (I was 36, and shooting to keep my heart rate around 140 bpm). That kept me going slow enough to let me maintain fitness while healing. Just a thought.

Also, if you're doing the five minute warmup walk, that should be adequate stretching. If you're going to stretch beforehand, it's important to avoid "static" stretching (i.e., stretching a muscle and holding it). There's some evidence to suggest that stretching cold muscles in this way actually increases the likelihood of injury. Static stretching after a workout is fine, as the muscles are already warm. Instead, "dynamic/ballistic" stretching is a better approach; things like brisk walking, easy jogging, a few easy minutes on the elliptical or stationary bike are possibly safer ways to warm up and stretch your muscles.

Good luck!

Don