Ummm, are you a runner now?? If so, how long have you been running? What is your current level of fitness, etc etc... More info Please!!
As a *General* rule of thumb, you *should* be a runner for approximately one year befor eyou start training to complete your first marathon. The reason is due to the amount of stress that running puts on your muscular-skeletal system and joints. The stress is not only from running the marathon ITSELF but all the running you will be doing to train for it. You should be able to *comfortably* run 20ish miles a week before you start to marathon train as well. Once again, i want to reiterate that these are just general rules of thumb. By no means are they the be-all-end-all rules.
I ABSOLUTELY think you should register for one, OR MORE half marathons before then!! Even some 10Ks! Races are so much fun, and they hellp you *practice* what is going to work for you on the BIG race day--i.e; what you eat the morning of and during the event, a routine that works for you, practicing your pace and all that good stuff!! IDEALLY while you are training you can find races with distances that approx. match what you are supposed to be doing on any given weekend. FOr instance if you could find a half-marathon on the same weekend that you are supposed to run anywhere from 12-14 miles, than that would be perfect!!
As far as training until may, i say work on building your weekly mileage, (your BASE mileage). Work on running consistently (like 3 days a week, working up to 4 days aweek) And gradually increase your mileage till you are running 20-25 miles aweek by the end of spring. At the same time you are working on increasing BASE mileage, also think about increasing the distance of ONE of your runs each week--your LONG RUN. Only run long once a week--whether long for you means 2 miles or 6 miles or 10 miles. your other 2 or 3 weekday runs should be of shorter (comfortable) distances. OF course, as your strength/endurance grows, your comfy distance will increase as well!! Right now you might run 2 or 3 miles a couple times a week with a long run of 4 or 5 miles once a week. By the end of spring your weekday runs might be as much as 5 or 6 miles, with your long run up to 10 miles.
Dont worry about speed right now.
I HIGHLY reccommend Hal Higdons BEGINNER MArathon or Half marathon Training program, once it comes time to actually start training.
If you ARENT currently running now, i recommend the COuch to 5K learn to run program. You can google it and find all SORTS of info!