Your slump describes my normal ride. I almost always HATE the first 10-15 miles, feel like I'm 30 pounds heavier than I am, slow slow slow, and have to fight the urge to just go home. After than, I'm happy, love pushing myself, and don't want to stop. Two weekends ago, I was just a slug, had to stop three times in 40 miles, when I often don't stop at all. I just chalked it up to being tired and the hot, humid weather. I took last weekend off because I didn't feel like riding. I got in a couple short, slow rides in the evening this week that were wonderful, and now I'm itching to get out tomorrow. Hoping the rain holds off in the morning.