I'm totally on board with Brett's assessment of the unavoidable condition of PRB. It's just really hard to stay focused for months and months on end. In my opinion, build blocks are actually easier than the race season. With these, you rely on very primitive survival skills, eating and sleeping enough to make it through the next day. Dying for rest days. Relishing in them. Doing it all over again.
Race season is different though. In theory, you are peaking and all your workouts between major races are more or less 'maintenance.' This is a terrible word that's used to describe awful interval sets that are designed not to make you faster but rather to hurt you just enough to keep you race ready and primed for the abuse. The worst part is, there's almost no chance you'll get faster (maybe 1-2% if you are racing on a 2 to 3 week cycle). There is a great chance you'll be unhappy and irritable though.
Anyway, if you're struggling through PRBs, let me suggest something that helps me get out of bed in the morning to complete these silly, silly workouts. PBR. Just kidding. Drinking is no way to fight depression. Caffeine is though. There are just so many great ways to take your coffee these days. I've recently gotten into Powerbar caffeinated gels, tangerine flavored more specifically. These little babies are my hummingbird food. Try one for breakfast tomorrow and see how your day just flies by!
Maintenance. Only the scrappy will survive.