You have a “lack of speed” because you don’t develop it at all. I would seriously consider skipping a fall marathon. Just find a bunch of short races and train for them. See how much you can drop your race times over the course of a season. Not only will it be fun, but you can gain more racing experience than just 2 marathons per year. You might even make some friends, some enemies and some training partners.
I set my marathon PR in the fall. I did a marathon in the spring and raced short all summer. Then at the last minute I decided to run a fall marathon. I threw in some long runs and viola, “instant” PR.
Regarding coaching: Anyone want the job!
Like Ryan has suggested in the past, it’s probably best to find someone local that you can have face to face meeting with. If you can get with a training group, I think that’d help too.
If not and you’re set on doing another marathon in the fall, I can email you some of the principles behind my training this time around. I won’t be able to tell you if they work or not till after Grandma’s though.