Hi TryonCreek,
Let's say my "add 30 secs minute to your 10k pace to get your marathon pace" target is too tight for everybody. (And it works with athletes I coach, even +27 secs/mile has been achieved).
Let's say 30-40 secs/mile added onto your 10k pace is valid for everyone under 3.15 marathon.
But you are saying you need to add 56 secs/mile (from a 6.00m/m 10k pace to a 3.02 marathon which equals 6.56m/m) and asking if this might be due to your large frame size.
It COULD BE, but it isn't. Because you state that you could still run 6.15 for 16 miles. Which is as good, or maybe even a hair better, than I would expect from someone who runs 6.00m/m for 10km. So there is little wrong with your training FOR THOSE DISTANCES.
BUT, as you don't need to be told, having run 10 of them, marathons are different. And they are MORE different for big guys (and gals). Because they have a unique problem all lesser distances do NOT have; the problem of fuel.
If you have to slow up 56 secs mile from 10k pace to marathon pace then in all your training, you have not trained your body to be fuel efficient. You have the physical ability to run (let's say) a 2.55 marathon, but you have not got the fuel mix right and are burning too high a percentage of carbohydrate. And, as you have learned, you cannot carry enough carbos on-board to get you past 22 miles.
So, you need to alter your "training mix" to improve your fat-burning ability. With the race being in April, you need to get in about 6 weeks of runs at M+40 secs and M+20 secs, and then about 4-5 weeks of runs at M+20 secs and M-pace.
Do one run at each pace per week, and include one long run per week. Aim for a marathon target (M) of 10k pace + 40 secs mile (just to be cautious).
A week at first might look like this:
Sun: Long run
Tue: build up to 75 mins at M+40 secs/mile
Fri: build up to 60 mins at M+20 secs/mile
At end Feb, a week might look like this:
Sun: Long run
Tue: Build up to 75 mins at M+20 secs mile
Fri: build up to 60 mins at M-pace
If you are a heart-rate dude, you are really wanting to get to a stage where the HR at these paces does not rise, but remains pretty stable (after the first 15 mins).
When I say build up, I mean you can start with 2 x 25 mins, and build up to 3 x 25 mins or 2 x 40 mins before reaching a single effort of 75 mins.
The rest of the week can be M+1.15-1.30 mins/mile (for a 6.40 marathoner, that would be 8.00-8.15m/m).
For the long runs, you might want to try running Sundays of: 1.45, 2hrs, 2.15, 2.30 (and repeat sequence).
You should manage your sub-3hr target on a max of 90mpw.
All of this is just calculated to improve your body's ability to burn fat for fuel. You are already "fast" enough to go sub-3hrs. But you are not fuel-efficient enough. Nail that, and you're there.
Good luck in Boston.