Agree with the advice to do intervals and especially learn what pacing works best for you.

Back in April a few of us did an impromptu local century route with 8,000 feet here locally. I was using my heart rate to pace myself, not letting it exceed mid 150s while climbing. I was done at 90 miles and 6,100', I had nothing left. Two weeks later I did another century with 11,500'. I used mid 140s HR as my max on the climbs and finished the ride feeling good! My fitness level didn't change much in two weeks, but pacing made all the difference.