Hi Electric Sheep (prepare to be called ES from now on!), welcome.
I would say about 1:22 or 1:23 for the half is more usually reckoned to be about the mark to do a sub-3 marathon, but on the other hand a few people are "abnormally" quick at the marathon. Most people are slower than the performance prediction calculators would suggest for the marathon when based on times for halves and lower though. So from that, sub-3 might be a little out of reach on your present form...
But... it all depends how much quicker you get through increased training. And how "trainable" people are seems to vary quite a lot. You might get quite a lot quicker as you ramp up your training and bag a sub-3 easily. It's hard to say until you try, and you're quite right (point 2) that it is a useful marker to do some half marathons along the way. And yes, your performance in those shorter races should really benefit too. There have been some stellar performance climbs by some members of this forum (in Marigold's case, to elite levels, correlating not just with heroic training but also shedding a lot of surplus weight, which can be a bonus by-product of increased training volume and can have a large performance effect).
The cliché is that a marathon is a race of two equal halves: the first 20, and the last 6. Seconds, or minutes, are gained or lost in the first "half"... but minutes, tens of minutes, or even an hour can be lost in the last few miles, depending on how badly you blow up. The last few miles of the London marathon look like a battlefield, with scores of 'walking wounded' even amongst very respectable performers. So developing your endurance to the point where you can bang out 20+M runs without too much trouble is absolutely key. My own preference is to do one or two runs of the full 26M or more in training, but most wouldn't bother. Certainly getting up to at least (say) 22 or 24M in training is pretty conventional.
And keeping your pace to plan for the majority of the race -- when it should feel ridiculously easy for many of the early miles -- is crucial. Think of it as a light 6M warm-up and then a 20M race.
Marathon pace miles in long training runs are tough; too many may just dent your performance, but doing some of your long runs with a fast section in the latter part ("progressive" run) is important to get a realistic idea of pace and (in my case at least) to build some confidence in what is achievable.
Nutrition-wise, I use about 4 caffeinated gels now, Tim Noakes' theory being that they should prop up blood sugar towards the end, even if they can't help your legs much, so avoiding a catastrophic descent into hypoglycaemia. If nothing else, they can be a welcome psychological boost. I find I can't stomach much in the way of sports drink but sometimes try some of that too if it is on offer. If anything, the more experienced people seem to use less, maybe being more conditioned to the endurance.
The "P&D" book (Pfitzinger & Douglas, Advanced Marathoning) is well-regarded, though they seem a bit light on truly long runs given my own bias.
