Knees Back Oh My

I am running in a 5K race this Saturday. It will be my second race in January. I signed up for 2 more races in Feb. ,
one 5K and one 10K. Unfortunately my February 5K race was cancelled- reasons unknown.. I'm a little bummed out about it. I'm loving those 5K races - they are short and fast, easy to train for, and are fun!
I'm still continuing to have mid, upper back strain while running. It seems to be a conditioning solution though. The farther I run, the more the pain kicks in toward the end of run. It use to hurt at the 1 - 1/2 mile mark, now at the 3mile mark. Okay 1 more gripe...My knees are killing me. I know it is due to my weight and the amount of running I am doing. I want to increase my mileage, but my knees don't. I run 3times/week now and want to incr. to 4.
Stick to the plan and keep losing!