From what I've been reading recently about endurance training, it sounds like lots of the advice being given here is wrong.
It seems that "No Man's Land" medium-level (of lactate threshhold) training is pretty useless for your long runs.
The optimum type of training is apparently lots of VERY slow distance running, at a "conversational" pace, mixed with VERY hard, short (no more than an hour) maximal sessions (probably only once a week of this for mortals).
Tons of evidence that this really works, and all the elite athletes have known this for years.
I'm finding it really hard to adjust to the long sloooooow though, as it instinctively feels as though I'm slacking. This is apparently a common reaction & must be resisted! hence the usefulness of heart rate monitors. (not that I've got one yet...or a Garmin
)