I have this problem too! There is one road on my commute to my soon-to-be-former job that I HATE for just this reason. It takes me twice as long to get up this "flat" road than it does to get up the huge hill at the end of it. The thing that works best for me is to think about anything but cycling. If I start thinking about dinner, or if I have time to take the dog to the park or how should I get my hair cut next time.... anything. The moment I look at the computer or think about how much of the road I have left... it's all over. I get tired and I slow down. I think I might just find it boring... but I do know that it is all in my head!