I am pleased to say I successfully completed the Luton Marathon last Sunday in 3.36, which is a new PB for me. After the half PB I picked up the last 6 weeks of the marathon training plan and all went perfectly.
Hi Cookie, if you're getting bored, I don't see any harm in mixing it up. Try being constant steady speed for the first half of your run then really pushing on for the second half. Or variations of that. I did my best half marathon and marathon time doing negative splits, and you get a boost when you're generally overtaking others at the back end of a race.
I admire your energy and motivation. Not to sound like a pessimist, but I would raise 3 points. 1) you're never going to have a break? No holidays? 2) you need to build up to the mileage you're proposing, going straight into 36 miles in one week (then every week) is a lot of mileage. 3) if you're going to be so dedicated over 4 years, why not build on the distance you're going to do and set yourself bigger goals. For example, in a dedicated 4 years you could really build your endurance and do a number of the following 5k, 10k, 10miles, 1/2 marathon, full marathon and an ultra.