Injury prevention!

Hey everyone,

So I have just signed up for my first marathon. I have been running for a number of years as well as competitivly swimming, and had previously been training for a marathon a few years ago. I had just done my longest run in the build up to it and was all ready when I got terribly injured and had to withdraw from it. Since i've had on/off problems with my right hip, but it's a lot more manageable.

My main question is how best can i prevent this happening again!?! My worries are my knees and hips which I know can be a bit problematic. I do all the obvious - stretch lots, do strength training, do non-weight bearing exercises (lots of swimming!) and eat well, and am not overweight...but I just need any other advice on how to avoid getting injured again! Any new stretches that might help (the ones I do are really just the basics) or vitamins or anything really! 

Thanks