WTF I keep getting slower and slower on the bike

So anyone have any ideas as to why I get slower and slower on the bike? I've been doing the same ~30 mile trail for all my training and have been doing it in its entirety. Each time I do it, my time increases by about 10 minutes. Recently my computer broke and I've got a new one coming in, so perhaps not being cognizant of my speed/rpm and just riding willy nilly is the cause. Anyone else have ideas? I'm not overtraining, I'm actually following a training schedule which includes days off. I have a sprint in 3 weeks and am trying to

I start out slow, spin my legs to warm them up, then after about 2 miles or so of that, I go about, or aim for 20mph for the rest of the way with (as needed) 30 second breaks for water/food/soreness in which I will go about 12mph at a high rpm.

It's not overtraining as I incorporate days off and alternate between bike, brick, runs, and swim. Any pointers would be appreciated!