Brighton 2016 will be my first ever marathon. Whether it'll be my last I'll let you know post race. I'm 50 in a few weeks so first marathon at 50 seemed as good a reason as any.
HM times suggest 3:10 doable. Nudging towards 3:00 would be amazing, but I'd have to improve my injury record of the last two years to make that remotely possible. As long as I don't blow up, my minimum achievement is 3:20 which'll get me in as GFA at London in 2017, but see second sentence of second para
I too am on a "running break" as I turned my ankle over and sprained it 9 days ago ... It's actually recovering very quickly though and though I'd written off the rest of the year I think I may get a few runs in between Christmas and New Year.
I think you're probably right on the HM front - my mileage is up and has averaged over 25 miles a week since the middle of March, but my longest run (ever) is still only about 14 miles - I think a bit more over-distance training would really help the HM endurance.
Impressed with your 5k and 10k times - well done The statistician in me can't help but wonder your age and hence the Age Grades of those two PBs. All my current PBs, except the HM set a few days before my birthday, are at age 48 giving AGs of a little over 75% at all events.