Here is an excellent question in from the UK: So, here am I, a 3:08 marathoner wanting to go sub-3. I’ve run 10 marathons, so presumably I don’t need more endurance, I need more stamina, which I’ll get from doing lots more goal-paced (6:52) miles. What then, is the place of the long slow 15-20 mile runs? Aren’t they buying me more endurance? (And while I’m on this tack, what, really, do the short easy pace runs I do between the more serious workouts accomplish?)
Congrats on some great times. You do hit it on the head. You need stamina and not more endurance. Your times will drop from doing more race pace specific runs. Your goal runs have to progressively become a larger percentage of your total miles each week.
There is a balance however. It is not physically possible to run ALL your miles at goal pace. However, since the overall pace of training is a better predictor of finishing times in the marathon then your overall pace (all runs combined) has to increase. That is done by making some of the easy miles become goal paced and as you do those long easy runs, to also put in some goal paced miles.
The other approach which helps your stamina is training your body (and mind) to run hard while fatigued. That can be done be doing the last mile(s) of selected training runs hard. For instance, run your 18 mile easy long run this way: 15 easy then 3 for time (HARD). Or, you can do 13 easy and 5 at goal pace. Here’s another approach that trains you to control your pacing. Run the first 5 miles (while fresh) at goal pace – NOT faster – then go for the next 13 easy. In any event you have infused your slower miles with modest amounts of faster miles which is what you need.
Especially for someone in your case it is very clear cut that endurance (longer long runs) is not what you need. But that does not mean you do not need long runs to succeed at the marathon. Therefore, those 15+ milers all build (in the early stages) and then maintain (in the latter stages) your endurance for the marathon. That being said too many age-group runners run too many of these runs. Most would be better served with more goal paced runs.
As for the easy run there are multiple purposes. One is recovery. You move your body, loosen up, allow blood flow to flush your body from harder runs. Another is building muscles and connective tissue and general running specific strength. You train your neuromuscular system to be a runner. That is why cross training isn’t as effective in this… it’s not neuromuscularly specific. [Doing the elliptical machine makes you a good ellipticaller… not runner.] It also is for mental reasons. Consistency in training, stress relief, you name it most runners rely on their running. I know this will sound contradictory to what I previously stated about overall pace of training but the key is not to run these too fast in order to accomplish their goals. The purpose of these is recovery. That cannot be done if you pick up the pace blindly on these. You will end up with “tweener miles”. There must be purpose to the workout.
All that being said, the easy run is the most expendable of your workouts and if you have to cut something – that is the one. That is how I can have 2:40s marathoners on 3-4 days a week of training. But, every workout is very targeted and quite intense in comparison to the 5 or 6 or 7 day-a-weekers.