is this your first marathon? if you're at 15miles for a long run, and you have 13 weeks remaining (i'll assume your race is the first Sunday of May (5th), for arguments sake) then you have time to build up to a max, as mentioned above, you should avoid running 20milers in consecutive weeks, as it'll take a lot out of you.
I'd also suggest maybe a 3 week taper, so drawing a straight line between this sunday (10th Feb) and three weeks before race day (i.e. 14th April, if race day is 5th May).
You could then aim to run a 20, 21 or 22 mile run on that day (if this is your first marathon, perhaps be conservative with the long run mileage).
Make a plan that has you running longer each week, peaking at 20 or 21, and dropping back every second or third week, so something like this:
16, 17, 18, 20, 17, 20, 13, 20, 17, 21, 16, 12, RACE
That SORT of thing, play around with it a bit, and it'll help if you stick in some miles at your target marathon pace at the end of SOME of the long runs.
i stuck in a 13 for example, to perhaps run a half marathon instead of a long run on one of the weekends, to give you a bit of race prep, and break up the monotony a bit. Just don't race it so hard that you're screwed all week and not fit for your other runs. Or perhaps if you are going to race it flat out, shuffle about the 20 miler the week after, or make it an 18 miler or something.
get the idea?