If you're already running 15 miles a week and you've done a 10K, you're a great candidate for a 10-miler. I wish all my friends who wanted to run 10 were as prepared as you!
I just did a 10-miler training plan for a friend. Each week, increase your current long run by 0.5-1 miles, depending on when you start training. Ideally you'll increase your total mileage or your long run mileage by 10% or less each week. And you want your long run to be no more than about 1/2 of your total mileage. (So do 2 midweek runs that are shorter, but ideally you probably want to get up to about 20 miles a week total.) sometimes after a really long run, I like to drop the long run back a bit the next week.
So for example, your long runs for a 12 week training plan could be 5 miles, 6, 7, 8, 9, 10, 8, 11, 8, 12 (longest run), 8 (taper), 6 (taper). But you can play it by ear depending on what other commitments you have during your training season.
Your longest run mileage can depend on your goal. If you just want to finish the race, get up to 8 or 9 miles in training. (with your longest run 2-3 weeks before the race.) The race day momentum can get you to 10. If you want your 10 miles to feel a little more effortless, get up to 10 miles in training. And if you want to really work on on a great time, get up to 12 miles in training for a 10-mile race. make sure you taper for a week or two before the race, with shorter long runs, like 7-8 miles and 5-6 miles. Then take it a little easy the week after the race, as your legs should be tired.
Also, if you don't get into Cherry Blossom (it's a lottery), sign up for the GW Parkway 10-miler. It's a great race. Army 10-miler fills up instantly, so be prepared to sign up at the time it opens on the day it opens.