I completely agree with Wise Old Man - you earned your spot there and have the right to run the race as you so choose. I loved seeing the women who qualified for the Trials and then became pregnant running at the back of the pack - they EARNED their spot, they didn't take it away from anyone else, and they had the right to enjoy their accomplishment. Whatever your circumstances are on race day, or however you wish to run the race (provided you don't impede anyone else) - you earned that right. If I qualified and were injured, I would line up and run the first 100 meters and drop out if I had to...for the "hobby joggers" who make it to Boston, they have the right to run it however they choose. Some people do it as a victory lap for the accomplishment of qualifying, some do it just for fun, some choose to race aggressively. To each their own.
Personally - I'd run with the lead pack as long as possible and be part of the race. You get a front row seat to see how the early part of the race unfolds, maybe you get pulled to a time you didn't think you were capable of, maybe you see you aren't all that different from the guys who wind up winning. There are races to do for time and there are races to do for the experience - I'm nowhere near that level, but if I were in shouting distance of the leaders for the first hour, I think that would be thrilling.
My fast days may be behind me but if I got in good enough shape to have a sub 3 marathon not be a pipe dream, I would definitely aim for that over a conservatively paced, smart race that falls between 3:03 and 3:06 and never has a chance at sub 3. I've run fast times before, but a sub 3 at my age is a much bigger deal to me than running a smart 3:04! If I blow up, I'm old enough to know that the worst races are usually the funniest stories to remember years later!