So what's up with the running?

I have tossed running references around here and there and perhaps mentioned that I am running the upcoming Mardi Gras Marathon (which is now part of the Rock 'n Roll Marathon organization, and therefore appears to be becoming a major mess). But I almost never write about running: I'm not really a runner, I just run for exercise. I'm not fast, and I'm a big old wimp. If that weather is ugly or there's cake at home, I'll skip. I do enjoy running, but I like many forms of activity because moving around burns off some of this excess energy bubbling forth from my irrepressible soul.
But in case you care, here's what's up on the running front:
In winter of 2008-2009, I hurt my knee preparing for a half-marathon. It turns out that this was due to wearing shoes designed to correct pronation, when I actually my gate is self-correcting for a leg-length anomaly (minor scoliosis). This is why they tell you to talk to your doctor before adopting an exercise plan. I continued to run, but by the summer the pain was keeping me awake at night. While training for the Jazz half-marathon I made an appointment with a sports medicine doctor, who told me I was in the wrong shoe and had severe tendinitis. I ran the half anyway, and I'd already registered for the full marathon in February 2010. I did buy new shoes, but I'm terribly disappointed in them. Right after buying them I ruptured my bursa (self-diagnosed but I think I'm right) and had to take several weeks off. I can't contribute that to the new shoes, but I have lots of other complaints about them. I've only put about 140 miles on them and they give me blood blisters and joint pain. The sides are already blown out and the tread is worn half off. I feel like there is no cushioning whatsoever: if I run over an area with acorns and sticks, I can feel each one digging into my sole. During fast runs I get blood blisters on the BOTTOMS of my feet!

Right now I am running with a knee strap on my right leg, which helps with the second injury, but not the constant nagging knee pain. Now I've adding hip pain and ankle pain - both of which feel like impact injuries. I've had to scale back on my long runs and frankly, I'm not ready for a marathon. The furthest I've run is 20 miles and my race is in 5 weeks; I did poorly for the 20 miles (it was boring, cold, and painful and took me far longer than I expected). I have a 30-k race next Sunday and I doubt I'm ready for that!
After the marathon I think I'm going to take a little time off running and have a heart-to-heart with my knees. Maybe I'll do spin or boot camp or something for a while.
In the meantime, advice on how to build my endurance and mileage in the upcoming weeks would be appreciated! At this point I'm not looking for time, just finishing without breaking something!