You don't give enough specific information to permit a definitive answer. However, what I understand from your post is that you are riding three times each week' that each ride is 2-4 hours, and (I'm guessing, here) you go pretty hard very time.
If that is right, then I'm not surprised you don't perform as well when riding two successive days. Recovery is important if you are to maximise the benefit of your training, and it will probably take more than 24 hours to recover from a hard three-hour ride in the hills. There are benefits, I believe, to going very hard for several successive days and then resting for several, your body "supercompensates" for the increased stress, but it isn't a strategy to play around with if you don't know exactly what you're doing. I don't, so I don't do it.
I'd probably recommend some more variety in your training, some easier days and some shorter but maximally intensive stuff. In particular, training as specifically as possible for the race by making one of your weekly sessions a full-on blast for 20km over the sort of gradient you'll encounter on the day. But frankly, if you're riding ten hours a week in the hills spread over three sessions, and pushing yourself each time, I'd say you were probably doing OK.