St. Eds Southside long run

Weather was OK: humid, cloudy, and 60 degrees F. I had to complete 20 miles as the advanced distance today. I ran by myself with my iPod talking in my ear. I tried to run easy as best I could. Over the run I ate three chocolate-chip cookies and one Clif-Shot as fuel. Nevertheless by the end my legs became fatigued and weak. Afterwards my legs were so tight that they felt like they were cramping. I made sure to stretch back at Rogue. I finished in 3:04:37 or 9:14 minutes/mile. So I did OK, but I’m really worried that I won’t be able to maintain 9 minutes/mile for the whole distance. This muscle fatigue is something that I’ve had before and I have been unable to solve it in the past. Is it something that I’m doing during the run (for ex, bad fueling, or trying too hard], or is there something I should be doing in preparation for it [for ex, strength training aka weights, or massages]. All I can say is that during the first three years of running I don’t remember this problem, but for the last ones it’s been a constant companion and caused me to “retire” from this distance. What am I to do, what am I to do? I’m worried.