Training Pace?

Loosely based on your PT time, you could expect to run about a 3:20 marathon. That would make your marathon pace about 7:45. Training paces for many of your runs would be 60-120 seconds per mile slower - or between 8:45-9:45. Tempo runs would be faster.

You would be doing yourself a favor if you started getting in some very regular running as many marathon plans assume you are starting with a decent base. If you're in the 15-20 miles/week range when you start training it will be much easier.

Here's my problem...

A 1.5 mile run is not very indicative of long course fitness.  In fact, most pace calculators do a horrible job going from short distances to marathon.

If you have something longer, or past experience training for a longer event, I'd go with that.  The calculators will give you a general idea as to what to use, but I wouldn't take them as gospel, either. 

The training calculators project a potential on the longer stuff "Assuming you do the work necessary to acheive it"

Why not use the faster short distance time to set up your training plan.  If your body cannot handle the work load you can adjust for that.  If you set it up on the slower lsd pacing, you may not be stretching yourself enough to achieve what is possible for you.

But it does all boil down to - do you want to do the work necessary and , can your body handle it?

Runner's World has a generic training calcultor that could give you a intial pacing to go from and the McMillian Calculator can give you an idea of the intensity/pace to target in various types of workouts.