are two answers to your post because you posed two situations, princess.
title of your post asked, ‘What is the best way to improve “marathon time”?’ But your post said that you and your co-worker disagree on the best way to improve "endurance". Although the two are related, they really require different answers.
best way to improve marathon time is to employ a balanced training program that includes all elements of training....and to
ratchet that program up in each training cycle. The best way to improve endurance
is to run as many miles as you can run without incurring an injury. The latter
is a very useful element of the former.
my opinion, the order of importance of the elements of a balanced training program for someone who has run more than one marathon
and is looking to improve as a marathoner:
Total mileage in the training cycle.
The number of 18-22 mile long runs.
would disagree somewhat with some of my order of importance. For instance, many
would place MP running much higher on the list.....especially since Pfitz's book was published 3-4 years ago. Others would suggest that max long runs of 26 miles or longer are important. Those "disagreements" are OK. There are few absolutes in running. What works best for one person isn't necessarily best for others. But, I think that most experienced marathoners would agree with the basic structure described above....the
most significant exception being the importance of MP runs.
does speedwork rank almost last? Largely because speedwork usually means running
intervals that are intended for developing or maintaining VO2max. Most serious
marathoners have already established their maximum VO2max. Thus, their need is
simply to maintain it, which requires a heck of a lot less emphasis in training than developing it does. That relegates speedwork to a secondary position in marathon training.
someone who is relatively new to running, regardless of whether s/he has elected to take on a 5k or a marathon, can gain great
benefit from speedwork. Why? Because
speedwork develops aerobic capacity (VO2max). And the factors that are most important
to how fast we can progress as runners, threshold and running economy, are a function of percent of VO2max. The old axiom of developing speed (read VO2max) before stepping up to longer distances has a lot of truth.
the case of your latter question....what is the best way to improve endurance....mileage is the clear answer. Speed training does little for endurance. However, it does
establish your aerobic capacity (VO2max), which is of "baseline" importance for marathoning.
Why? Because one of the ultimate determinants of how fast you run marathons....threshold....is
a percentage of VO2max. Thus, the greater your VO2max, the higher your threshold can be developed and the faster marathon