if this is your first marathon and you're used to running just 15 miles per week I'd start with the beginner plans, either Hal's or others.
All first time marathoners have to effectively guess their target time, since they've only got shorter distance races to extrapolate from, and it's not an exact science.
The RW schedules I think do have different plans for diff target times, but like stutyr said, concentrate on training well and let the chips fall where they will.
if you want an idea of what paces you should be training at, and guesstimates for target race pace (over different distances) check out...
http://www.mcmillanrunning.com/calculator
it's worth taking all advice with a pinch of salt though, as only you will know how well you're responding to training. Since most plans are somewhere around 16 weeks in length, you'll probably start training in earnest just after new year. for now i'd focus on getting your weekly mileage up from 15. this will mean more time on your feet, but you'll need this to strengthen your muscles and connective tissues to take the battering they'll get running 26 miles.
a rule of thumb used for increasing mileage is to not exceed 10% increase in total weekly mileage week on week. If you can absorb training well enough, and since your weekly mileage is relatively low, you might be able to take slightly more than that, but listen to your body and if you start getting pain (as opposed to stiffness or muscle soreness) then back it off, both in intensity and volume of training.