What I have heard regarding shoe life is for the normal weight runner, after about 300 miles the cushioning starts to break down. I listen to my knees. I replace my shoes when they bother me after a run. This usually takes 9-10 months, depending on how much running I have been doing. I probably average 10-12 miles a week when I am running.
X 30 weeks = 300-360 miles. So, this seems reasonable. Everything feels fine
during the run, but after my knees are tender. I have found this to be true for all brands I have worn. Obviously more or less running will change the amount of time that it will take before you need to demote them to lawn mowing shoes. One thing I would recommend when you get to actual marathon training is buying 2 pairs of shoes, and alternating them so that the cushioning has time to fully recover between wearings. A good friend tried this, and swears by it.
Halhigdon.com has the training plans I use most frequently. In general this is a great site regarding running info. You'll also get lots of hits by typing "10K training plan", or whatever, into Google. Skim these, and see what works for you. HH.com's plans have the long runs on Sundays. I prefer to do mine on Saturdays, so I shift everything by a day. I'll scan in the plan I used for last years 1/2. I print the plan off, put in in my workout area and use it as my training diary. Remember, this is just a guide. If you feel like doing more or less, listen to your body.