Sometimes you just need to take the first 5-10 miles a slower pace. I always need to do a long warm up, which is why I don't do well on a lot of group rides, which takeoff like a bat out of hell. Yet, I often find that many times, even though I might have felt like i was struggling in the beginning of a ride, when I ride with other people, I'm often right near the front at the end of a ride, and my climbing is really strong, too. It's always been like this.
I agree with NY Biker, too. Nothing makes you want to ride more than taking time off from it. I have less miles this year, due to weather, work, and now I'm recovering from a sinus infection that has totally knocked me out. We are leaving this morning for what was supposed to be a weekend of hard riding int he Berkshires; the weather has pretty much eliminated that, and we are not even taking the bikes. We might get some hiking in, and I know I will be really ready to ride next week.