Fascinating about the physiological versus endocrine responses. Any insight/advice about variability among runners in this area? For example, I seem to have mediocre races at the beginning of a season, respond rapidly to the endocrine-type work (for example, I even ran my 10K p.r. after a couple of hard mile-to-800 paced 600m sessions), and then get sick or injured. More or less as you described. Others seem to race well off basework, improve gradually, but not get that extra boost--and maybe even get worse--from those extra-hard workouts at the end of the season.
Thanks for sharing your knowledge and enthusiasm!