I have also coached youth and adult soccer in HI and WY and have a USSF Category C coaching license. I have experience with youth soccer in AYSO and USYSA, some high school experience and have managed several adult teams. At one point, I was appointed the Select Team Coach for the HI branch of USYSA (HYSA).
I have spent a lot of time reading articles on how to train and on technical advice in particular sports. I am not easily fooled by new 'best' ways of doing things but do give them consideration. The following paragraphs are a few of my thoughts.
Here are a few research articles from Jan Helgerud and his colleagues on the efficacy of high-intensity training versus more traditional high-volume, moderate-intensity programs. These training protocols are controversial. However, they were followed by the Swedish skier Charlotte Kalla when she first 'burst' upon the World Cup racing scene and won the second edition of the Tour de Ski in 2008 at age 20. The conclusion from the research is that high-intensity training is preferable to moderate-intensity training to bring an athlete to the highest VO2 max. This has implications for how much overdistance work an athlete should do versus the interval-training (or, rather, the high-intensity) alternative.
The first article by Wisloff et al (2001) sets the stage with tests on lab rats.
Intensity-controlled treadmill running in rats...
The next article by Helgerud et al (2007) describes the tests and conclusions for a group of male athletes.
Aerobic high-intensity intervals...
And, finally, Wisloff et al. (2007) describe the same exercise protocol for recovering heart-failure patients.
Superior cardiovascular effect of aerobic interval training...
Look these over and draw your own conclusions. What I get out of it is that we probably don't need to do all the long, slow distance that most of us are doing in order to get fit. Alternatively, we should probably be doing more training that gets the heart-rate elevated.
Noted psychologist K. Anders Ericsson has studied what it takes to become an 'expert' in musical performance. His work has been generalized to other highly-technical endeavors, including sports. Below is an excerpt from his faculty page at Florida State University:
"For example, the critical difference between expert musicians differing in the level of attained solo performance concerned the amounts of time they had spent in solitary practice during their music development, which totaled around 10,000 hours by age 20 for the best experts, around 5,000 hours for the least accomplished expert musicians and only 2,000 hours for serious amateur pianists. More generally, the accumulated amount of deliberate practice is closely related to the attained level of performance of many types of experts, such as musicians (Ericsson, K. A., R. Th. Krampe, and C. Tesch-Römer, 1993, ‘The role of deliberate practice in the acquisition of expert performance.’ In the Psychological Review, 100: 363-406), chessplayers (Charness, N., R. Th. Krampe, and U. Mayr, 1996, ‘The role of practice and coaching in entrepreneurial skill domains: An international comparison of life-span chess skill acquisition.’ In The Road to Excellence: The Acquisition of Expert Performance in the Arts and Sciences, Sports, and Games, K. A. Ericsson, ed. Mahwah, NJ: Erlbaum, pp. 51-80) and athletes (Starkes, J. L., J. Deakin, F. Allard, N. J. Hodges, and A. Hayes, 1996, ‘Deliberate practice in sports: What is it anyway?’ In The Road to Excellence: The Acquisition of Expert Performance in the Arts and Sciences, Sports, and Games, K. A. Ericsson, ed. Mahwah, NJ: Erlbaum, pp. 81-106)."
For me, the takeaway from the above is emphasized in bold: the time they had spent in solitary practice. Let's consider the serious amateur who needs to spend 2000 hours in solitary practice to master a skill. For a skier, this does not mean social skis. You must just go out and practice by yourself and not be distracted by your surroundings. Using myself as an example, I train 500-700 hours per year. Let's say 600 hours. I ski roughly 1/3 of the year, so that is 200 hours per year. If I do nothing but repititious practice to master the skill (can't count the social skis, races and high-intensity practice days...), that's more than 10 years just to become a serious amateur. It's more if you allow for some down time. If you want to do it, there should be no junk kilometers and no random, unfocussed skis. You have to think about every step and figure out if you really are doing what you should. If you are not doing it right, you have to find an expert to help you and practice what that expert advises.
Most of us will just throw up our hands and go out and ski. We take whatever comes from that and just enjoy ourselves.