Dproven- this is all just based on my personel experience but I found the thing that effect my long runs were
1) Consistency- my weekly miles this year have been 61, 62, 50-recovery week, 70, 71, 72, 50-cut back week, 73(this weeks target). Another words no erratic weeks.
2) mileage- this years weekly miles have been much better than last years 40 a week average this has ment that the long run of say 20M doesn't represent a massive and quite frankly overwhelming 50% of weekly miles, less than a third now, so the first 2 hours of a long run feel relatively comfertable

3) body weight- 18months ago I was 13st 6 and doing 8min pace was pretty hard especially in summer, now at 11st I can cruise along at 7min pace fairly comfertable.
4) also I drink copious amounts of tea so really dont need to take anything out on training runs to drink- though I probably will in summer.
Hope some of that helps.
Andy.