Like others have said, you may have just had a down day. The weekend before this last one, I did a loop overnighter with lots of steep climbing. I rode very, very slowly on the last big climb to the campground, but made it. So this weekend, I did a much less hilly loop of the same distance (just got back), but seemed to just have no real "get up & go" both days, compared to my energy level on the hilly trip, even with a somewhat lighter load on the second trip. (I'm factoring in the temperature difference, we have a big heat wave here at the moment, but I did camp at the much cooler beach)