Stimulated by my own frustrating inability in recent years to run as far on my weekend "long runs" as I could just 3 or 4 years earlier I began to search for some physiological reasons. For years I would run a slow 18-20 miles on Saturday or Sunday throughout the years but in last 2 years I have basically crashed around 14-15 miles and was generally wiped out the rest of the day loosing weight from a pre run of 160 to a post run of 152 to 154 in warm to hot weather conditions and having a postural BP decrease indicative of volume depletion.
So far I have found evidence in the literature that at least 4 factors might play a role
1.A age related decreased in nocturnal ADH ( Arginine vasopressin) may cause a nocturnal polyuria.
2.Increased rate of glycogen utilization in older runners.
3.Lower levels of body water in older people.
So that my body water in the morning was lower to begin with as more water lost in the evening preceding the run and my glycogen stores were more rapidly depleted even after some efforts at glycogen loading leading to some limiting level of dehydration and glycogen depletion.The relatively meager liver glycogen stores are used during sleep.
4.Decreased V02 max with aging. the carbohydrate (CHO)/fat mixture changes as a person exercises more intensely and at some percentage of 02 max ( often quoted as about 60%) fuel source shirts to mainly CHO and little fat so that CHO stores ( glycogen) are more quickly depleted).So maybe in a misguided effort to run a bit faster than I should be,I get glycogen depleted quicker, i.e. after fewer miles have been run. So lower V02 leading to quicker glycogen depletion.
Dehydration in the older runner likely leads to to lower blood pressures post long hot run as the stiffer ventricle and central arteries respond to decreasing blood volume with exaggerated blood pressure changes- sort of a Starling curve thing. This of course is an effect and not a cause of volume depletion.