Here is another JK posting that is very helpful:
There are benefits from several types of long runs Posted 10-16-2001 17:43
Slow runs:
These should be slow enough so as to give the feeling of "storing up energy" (Ernst van Aaken called this pace a "super-O2" pace). The only thing which might present any difficulty is the sheer time spent out there; otherwise, the effort should be completely conversational. Relaxation is paramount. Water and/or "road race" fluids should be taken every 20-30 minutes (depending on weather conditions) to stay hydrated and to practice taking race day fluids with ease while running.
Primary benefits include fatiguing "slow twitch" muscle fibers to the point where "fast twitch" ones must be mobilized to continue the energy output, increasing capillarization, improving mitochondrial density, neurological and psychological relaxation (which promote efficient running mechanics), incrementally (and, hopefully, SAFELY) increasing muscle and joint integrity, balancing glycogen/fat metabolism, and simply becoming familiar with the feelings of running long. The last 1-2 miles of such a run should be SLIGHTLY picked up to reinforce correct muscle fiber recruitment and to gradually train the body to handle increased carbohydrate metabolism when somewhat fatigued. A slight pickup at the end of a long run may also provide some psychological benefits.
It's probably a good idea during a pre-season to build these up in duration until they are just a little over two hours long (occasionally longer for runners who specialize at distances above 5,000m). While increasing the duration, the long run day should be treated as a day (low to moderate mileage and little or no intensity the day before, low mileage and no intensity the day after). For those who will race 2-4 times per month once the competitive season arrives, assuming the long runs have been at 2-ish hours a few times, the weekly long run can be reduced to about 90-95 minutes for maintenance (as Lydiard said, "Jealously guard your good condition ..."). At this point, the long run can come the day following a race (and probably the day before a moderately workout), since 90-ish minutes will no longer be any strain.
Accelerated runs:
These are more directed WORKOUTS of 90-120 minutes. Done in a manner most people would associate with Japanese/Kenyan style (although Americans also did them like this decades ago), they feature a very slow start (like the aforementioned super-easy runs), and aerobic equilibrium should be achieved long before any pace changes take place. This means breathing pattern, muscular recruitment, heart rate, perceived effort, and mechanical rhythm should all be stabilized (in simple terms, the pace should feel GOOD N EASY ). Once this equilibrium is attained, a very slight increase in pace can take place every 10-20 minutes until the final miles (sort of like a graded exercise test on a treadmill). The last few miles are normally run slightly faster than marathon race pace (fractionally slower than half marathon pace), with the ultimate 2-3 minutes REALLY MOVIN' (@ 10,000 pace). This may take lots of practice to get the correct feeling. The runner should feel "on top of" the pace (i.e., in control and NOT struggling) even through those last few minutes. It should only be a workout, not a race effort, and should be something that can be recovered from within 2-3 days. Again, fluids should be taken before, during, and after the run.
These workouts provide more stimulus for aerobic development than the really slow ones do, but - IF the overall mileage is adequate - they should not be done on EVERY long run day; otherwise, recovery time will be too long to integrate other aspects of training into the overall scheme. Besides EXPIDITING all the benefits mentioned for the slower long runs, these acceleration runs also improve running economy (the ability to use oxygen more efficiently and preserve stored fuels) and improve the ability to metabolize small amounts of lactate before a spike is reached and the "threshold" is crossed.
Pace runs:
These are usually reserved as "dress rehearsals" at target pace for an upcoming marathon race and can involve distances anywhere from 12 to 20 miles. Pre-race fluids/foods and in-race fluids are taken for practice (but no taper or special supercompensation scheme is undertaken during the days before), and the run is done as close to goal pace as possible for the early miles. The last few miles are gradually squeezed down to a faster pace. There should be 2-3 of these prior to the target race, spaced about 3 weeks apart, with the last one 3-4 weeks out from the race. These are very runs and will obviously require several days of easy running for full recovery.