weewheels - so long as you don't go crazy as suggested by others, you'll be fine.
To put it into perspective for you, last year I set myself a fundraising challenge of running 100 miles (2 marathons, 1 ultra (33 miles) and a 15 mile hill race). My prep for all of the above was absolutely terrible. I'd ran a half marathon a couple of years previous (1:54) and a handful of 10k races. I ran (well, shuffled) the 33 mile ultra in 7:17 on the back of about 4 or 5 long runs of about 12 miles (ridiculous, I know), did the 15 mile run in 2:20, again really just on 12-13 mile runs (maybe a couple after the 33 mile run), then the Edinburgh Marathon in 4:48 (in horrible heat) off the back of one 17 mile run and the rest 12 or so. I then had 4 or 5 months off - instead of training, I went back to football and then did about 6 weeks' training for the Loch Ness Marathon - I did manage a couple of longer runs, longest being 21 miles, plus a couple of 15 mile runs, but really nothing close to proper marathon training, and I ran that in 4:24.
All of those runs hurt, a lot. I've learned my lesson though - proper training plans are the way forward! Take it slowly and build it up as folk have said above and you'll be fine, and you'll get a decent marathon time! Good luck 