My observation is that at running club (not serious athelete) level, people running marathons are most comfortable off a large base of continous aerobic runs. You can only do the high mileage necessary to be comfortable at 3hrs + by running fairly easy.
eg 10-15miles is more specific that say 6x800.
I also observce that intervals over shorter distances benefit people running say 10K.
horses for courses as they say - or specificity of training.
See Frank Horwill and Seb Coe on multi pace training.
Some recent deaths in the UK were related to hydration problems.