I am using
Halhigdon.com's 15K beginner's program this year. Last year I used the HH.com 10K plan, which I posted (#16) in the "anyone here run a marathon?" thread. I have retyped it into Excel, to keep track better of what I was supposed to do vs. what I actually did. I'd be glad to email that to you if you PM'd me an address. My cross training is usually biking.
You are probably in good enough shape now, with all the soccer etc. because this 'race' really isn't. There are sections of running punctuated by a lot of waiting at bottlenecks from the creek crossings and hill climbs, so it's not like you are running the whole 7 miles continuously. Which, IMHO is part of the fun, cuz you can actually shoot the breeze and make fun of how dirty people are, or try to get your friends dirty...
If you are serious, I'm in week 2 of the 15K program. Would be great to have another running partner! As for your knees, go toa running store, and get a recommendation from them. It is worth it. My knees bug me when it's time for a new pair (after about 325-350 miles in them). You should have a pair used only for running.