Injuries happen with marathon training. It's a fact of life for some people. Luckily, and let me knock on wood here for a second, I have been fortunate to dodge most of the common runner ailments. I think the biggest reason I haven't been affected is that I'm a biomechanically efficient runner. I do not supinate or overpronate, my pronation is normal so my gait is neutral. That definitely helps avoid injury, but that isn't exactly the best advice since everyone's anatomy and gait vary. The most frequent runner injuries I have heard of or seen are:
1) plantar fasciitis - inflammation of the plantar fascia that runs from the heel to the forefoot, and a common overuse injury. I think I've had a mild case of it, but it hasn't bothered me lately, when it did bother me at first, I did some exercises like scrunching up a towel with my toes, and running the arch/sole of my foot over a tennis ball. The pain I had wasn't excruciating, and more directly posterior to the ball of my foot on the medial side, not under/around my heel like it commonly presents (or so I've heard).
2) shin splints - no experience with these. I guess I have tibialis anteriors of steel. You can see how they could be prone to be injured though since they can bear a pretty big load during dorsiflexion, and some people are more predisposed to these as well, but I haven't read up on why.
3) patellar tendinitis - I see and know a bunch of people that run with the bands around the knees to help with this. Again, I feel lucky to have avoided it. It's an inflammation of the patellar tendon that connects the patella (kneecap) with the tibia (shinbone). It's more common in activities that involve jumping, and is sometimes called jumper's knee - basketball, volleyball, some track events, but happens a lot in runners as well.
4) iliotibial band syndrome - I have no experience with this either, but it caused my brother-in-law a lot of pain when he had it a few years ago prior to the first Marine Corps Marathon I ran in 2007. He was going to run it as well, but the pain was too much and made running too difficult. The iliotibial band stabilizes the knee during movement, for example as the knee is flexed and extended repeatedly during running. It runs from the outside of the hip to the inside and just below of the knee, and commonly causes lateral knee pain in runners, but can cause pain throughout the tract wherever it is inflamed.
Muscle strain/fatigue is about the worst "injury" I have had over the past three years that I've been running, but that happens with any weight-bearing activity, especially distance running. The worst is quad fatigue after long runs when you have to go down stairs sideways, unless you feel like crumbling/buckling your knees sounds like an awesome idea, because you're wearing Bad Idea Jeans™. This week someone told me, "Running marathons is the worst thing you can do to your body."
I said, "I'm pretty sure smoking is worse for you than running."
Bottom line: my advice regarding injuries is to listen to your body. If you tweak something, you know yourself best. If it's something you can deal with and continue your workouts without it worsening, then try to keep moving forward. If it's something you can't deal with, get it checked out and hopefully have a speedy recovery.
My last bit of advice: it's fine to take a day off from time to time. You won't sabotage your training by missing a day if you aren't feeling it--physically or psychologically.
I'm crossing my fingers, cutting back my mileage and enjoying my taper. There's only 12 days until the Marine Corps Marathon! I'm still accepting donations but additionally will take any moral support or encouragement as well!