Ok, well if your 'engine' got NO FITTER WHATSOEVER, and all that happened was you were to lose weight down to 13stones, your 2hr08min time would tumble to 1hr49, that's 19minutes faster, just by your engine having to push less mass along.
Now, since Seb Coe had a VO2Max of 82, and a weight of 54.5kg's, his "engine power" was 4469 (weight x VO2max). So cutting to the chase, if his engine was in a 13stone body he'd run a half marathon in 1hr 36.
So weight will get you to 1hr49, and upgrading your engine to that of seb coe will bring it down to 1hr 36.
I'm guessing you won't (like the rest of us!) get your engine that good, so losing weight to 13st and getting fitter through intervals will give you something like the 1hr 40 you are aiming for.
Workings for maths available on request
