I'm looking at the plan, and the calendar, and I'm assuming you're doing your long runs on Saturday. Following the plan as written would have you finishing week 11 with a 20-miler on Feb. 22. (I've checked this, like, 5 times, to make sure I'm counting right.) That's 3 weeks before the race and gets you a 20-mile long run and leaves you with a fairly traditional 3 week taper. After week 11, do weeks 18, 19 and 20. The long run for week 18 could be up to 14 miles (if it will make you feel more confident) and for week 19 can be anywhere from 8 to 12 miles. Judge by how you feel and how well you recover from your other long runs. Tapering is difficult mentally but you want to be pretty well rested for the marathon (leaning slightly toward shorter in week 19). And, of course, the final 2-miler in week 20 will have to be on Thursday or Friday, since the race is on Saturday.
You should be in decent shape for the marathon. Since it is your first, run the first 15 to 20 miles fairly conservatively, like shipo said. This will make for a more "comfortable" race overall and and leave you in better shape for the final miles.
Good luck.
