Paced MCM last year - on a brutally windy day which ultimately caused me to miss my time (3:10:47 - still a BQ, but I wanted to be about a minute faster - had a BAD day!)
Because MCM is a very popular (read:BIG) course with lots of first timers, if you start with 3:20 you may find yourself pretty far back. I'd position yourself in between 3:10 and 3:20, erring on the side of being too close to the 3:10 group before the cannon goes off.
3:15 is ~7:27/mile - last year I was right where I wanted to be through the early miles. My goal time was 7:15 and my first 3 miles up the hills averaged around 7:30 - take it easy up those hills! The 4th mile you'll make up a lot of that time - we made up half of the 45 seconds deficit in that one mile with no effort at all, and the 5th mile is also downhill. Miles 5-8 are less steep but generally uphill - by this time we were warmed up and ran them right around 7:10-7:20.
From 8-10 is a nice gentle downhill so try to get back onto your overall goal time during these 2 miles - you should be fairly close. It's time to start banking a little bit of time - if you're a minute slow at mile 8, you're in decent shape - it should feel easy, you've relaxed up the hills and you can make up most of that time from 8-10. Use that momentum off those hills to keep rolling from 10-20...if you can bank no more than 5-10 seconds per mile from 10-20, you'll have a nice 60-90 second cushion - the bridge to Crystal City is tough, and there's a climb from 25-26, plus the last little bit is uphill as well. If you can get to 20 with that banked time, try to squeeze 1 more mile out at 7:27, and then 1 more, and then another. If you can maintain most of your banked time through 23, you can slow down up those hills the last 2 miles and still make 3:15.
Summary - take the first hills easy, roll down the hill from 3-5, relax from 5-8, pick up the pace from 8-10, try to bank between 10 and 20, hold on as best you can 20-24, and be prepared to give back your banked time from 24 on to the finish. Then go home and fill out your Boston application and have a beer!