Moraghan - given MP is just a slow tempo pace and if we believe tempo runs provide the optimal training stimulus to improve your lactate threshold what, in your view, is the training benefit of MP runs? I think there is often a danger that people get so obsessed with MP runs that they do them at the expense of tempo runs and receive a weaker lactate threshold training stimulus.
How did the faster tempo runs (1 hour race pace so we are clear) become so popular? I think (happy to be corrected though) that most of the original research on running to improve LT involved a control group and another group doing 20 minute tempo runs. So, the fair conclusion was that tempo running was a good stimulus to improve LT. But they never did compare the effectiveness of the 20 minute tempo run with the MP (for example) as stimulus so I would never, ever agree that the tempo was optimal.
Obviously there is a trade off between intensity and duration. I just believe that you'll get a much better training effect from a 8 mile MP run than a 4 mile tempo run. I believe for most runners the two are approx equivalent in overall effort but with the former you spend twice as long addressing your LT.
I can't help thinking that these famous 10 mile club burn ups in the old days are what made distance running so strong - and more than likely these were principally at MP as were Lydiard's base runs.
I think it's a major mistake to ignore any pace around the LT zone but that you have to cover all your bases between easy paces through to 10k (quicker for some). Put another way in the example of paces for a 3:15 marathoner:
Easy Pace: 8:15
Steady State: 7:45
MP: 7:27
Tempo: 6:55
10k: 6:40
Too many people ignore the SS / MP zones and alternate from easy pace to tempo. This only makes sense to the brain in terms of slotting into certain paces, not to the body. The result being that there is a gaping hole in aerobic background. Tempo runs have their place but in the past this has been overemphasised at the expense of other paces.
Truth is I don't much like tempo pace. I think it's the one pace in the typical range that can be missed out. I still use it but it would be more of a late season LT maintenance or even peaking workout or some "close to race pace" practice for a 10 miler. In a generic marathon schedule I'd do the bulk of the early work at MP and 10k pace (this helps the LT but also with a vo2 max component) and then finish off with some tempo work to tie together the more fundamental paces.