There are a number of different ways to calculate a marathon time based on a recent half marathon.  If you use one of the more common ones, you come up with a 3:20:30 marathon time for you.  That's based on your half marathon PR plus 10 minutes.

Since you need to run 3:05 to qualify for Boston, you'd have to have a REALLY good day.  My suggestion is to find a good training program and follow it.  And work on fueling, you'll find your biggest obstacle on race day will be avoiding the wall and you do that by pushing it out past 26.2 miles.

I had the great idea of trying to qualify last year, cranked my mileage to 70 a week while doing weekly tempo runs, 1/2 mile interval sessions and long runs.  I kept this up for weeks, I'd read that getting your mileage to 70 a week would get you to a new fitness level.

I had a good taper leading up to the race and carb loaded for 3 days.  After all that, I cramped at 19 miles on race day and could  barely walk.  I finished the race because they'd given me an awesome tech t-shirt I wanted to wear, but I paid a heavy price.

I've had good marathons, so they're not all bad, but you might consider running one for the experience before going after a qualifier, there is a learning curve you have to go through.  Just my 2 cents... 

In my case, I was over-trained going into that marathon.  There has to be a balance, you have to train enough, but you also have to rest and let your body adapt to the training.  That's where I made the mistake, I wasn't using anyone's training program, just winging it with no rest days and high mileage.

You probably already know the average person has enough glycogen stores to run about 20 miles.  Without fueling as you run, you'll run out of fuel at about that distance and hit the wall.  You can still run, but your body is trying to burn fat for fuel and it's not efficient enough to keep you running at full throttle.

So... the way to push the wall out past 26.2, is to make sure your glycogen stores are at full capacity (by carb loading in the days leading up to the race) and by fueling DURING the race.  Energy gels are what most people use, experiment with them during your long runs to find what works best for you.

Another thing to keep in mind is pace.  Your body is like your car, it'll burn more fuel the faster you go.  If you start out running at a pace where you're working, you'll burn fuel more quickly than if you settle into an easy pace.  It's one thing to run a marathon just to finish and something else if you're racing 26.2 miles.

I hope this helps, but I'm no expert at this.  I've got the speed to qualify for Boston, but so far haven't gotten the training right.

And like I said, it wouldn't hurt to run one just for the experience...