Improving longer distance?

I do fairly well in shorter races but when I get much over an hour I fatigue, (esp legs) and my technique suffers, and some of the people I worked so hard to pass return the favor. Any general suggestions for improving more in longer races?
background: fiftyish master skier, 6 years racing, regular skier for 10, gradual SLOW improvement, weak right leg from old ACL tear and reconstruction (done in the days before they had the operation fully worked out) Have always had good general strength but never much of an endurance guy prior to last 10 years.
Thanks