I have much better luck doing my surges/intervals/sprints - whatever the geeks on here want to call them - during the last half of my rides. Up to about 100 miles. On rides over that, I don't do them at all, just save my strength and try for even effort. I takes me about 2 hours of riding before my HR will come up. I led the group up a long hill today, about 40 miles into a 50 mile ride, holding a steady HR 5 beats below my max. No one came around me. Heh. But I got dropped by two people on the first climb of the day - aerobic system wasn't ready yet. My theory is that it's hormone production. It takes that long for my system to get saturated with go juice, whatever that is. I did do short hard efforts during the first half of the ride. That helps me to prepare for the harder efforts in the second half.