In training for distance running our work is centered on the stress and recover principle that states that if we stress the body in a certain way (running), and then let it recover, it will be better adapted to that stress than it was before. I call 1 stress and recover cycle a "base unit". So in training we increase our fitness by stacking base units, one on top of the other, for uninterrupted periods of time (a training block). The mix of stresses (workouts) we do will depend on the event we are training for as well as many individual factors.
Because we have multiple different systems to work (cardiovascular, muscular-skeletal, energy, neurological, etc.) we need to be able to work on each on a regular basis, so adaptations made in that system don't regress before we work on it again. For this reason we need to arrange our base units and training blocks in such a way as to accomplish this. One key to doing this is to properly managing the length of our base units. We want to find that balance where our stresses are hard enough to produce the adaptations we are after but manageable enough that we can recover from them in 1-3 days in order to be able to handle another stress workout (often focusing on a different system). If our base unit is too long we won't be able to get to all the systems often enough and if they are too short we are either not stressing our system hard enough or get good adaptations or we are cutting our recoveries too short and this leaving some of the adaption (super-compensation) on the table.
Complicating this whole balancing act is the fact that distance running is an impact sport (causing more strain on the muscular skeletal system, which is the slowest to recover) and because of this it requires more recovery time than its other non-impact endurance sports such as biking and swimming.
Given everything I just typed, then our main focus on our stress workouts is to work hard enough to gain a good amount of specific adaptation, but not so hard as to required extended recovery time. This effort level is usually pretty hard but not an all out (race level) effort. I call it a good hard 95% effort, in which we finish the workout know we worked pretty darn hard but that there was a little more left if we absolutely needed it. If we do this we will get good adaptations and benefits from the workout but still be able to recover from it within a couple of days and do another stress workout of a similar difficulty.
Side Note: This concept (95% effort workouts) is especially important for longer distance runners (such as marathoners) where all out efforts require more recovery time. Middle distance runners can get away with going all out in a workout more frequently than a marathoner can as they recovery from doing so for them is not as long. I think this concept is one of the main things many coaches and athletes struggle with when they are trying to transition from the collegiate system, focused more on middle distances (800-5k), to the half marathon and marathon distances, they are use to pushing to the limit in most every workout and that rarely works well training for the marathon.
We can definitely have training paces we target to help us find this right level of work on these stress workouts, but the paces should just be a tool and the proper effort level be our primary goal. Hitting a pace doesn't ensure the desired adaptation, but the right effort level does. I have done extensive work on what kind of paces we can expect to hit in workouts given this effort level at certain race fitness levels, but it is only a guide and not the ultimate driver. This is why some elite runners, guys like Steve Jones back in the late 1980's, could train effectively without a watch, they were in tune with the desired effort levels for their work, which ensured they got the benefits they sought.
On the flip side of the stress workouts is the recovery runs we do between them. They are just as important to our success as the stress workouts. These recovery runs need to be done at an easy enough pace to recover from the stress workouts and so the adaptations (super-compensation) can occur, but at the same time not so slow as to teach our body bad bio-mechanical habits. Similarly to the stress workouts, I have done extensive work finding what I think is the type of paces that accomplishes this for each race fitness level, but the key is always going to be recovering on these runs, so pushing the pace on these runs does no good.
Note: given this approach (95% effort on stress workouts) it becomes important to schedule periodic races (once every 4-8 weeks) or time trials into your training cycle, where it is the goal to give 100% efforts. These 100% efforts are needed periodically in order to stay mentally calloused to those all out efforts. It can be hard for some people to give 100% in a race if they have not done so 6 months because of lack of racing.
I hope that this answers some of your questions on efforts in training and how it relates to what we are trying to accomplish.
Coach Mark Hadley