It just happened to me today. TWICE.
I woke up with a 2 hr bike on my computrainer(in my garage, that holds my race bike in place while i ride creating pretty much the realist possible way of riding outside since you can adjust resistance, power, etc) and a 4,200yds swim (in a pool ;) on tap for the day.
The bike, that was just mental really. Your long workouts my friends, you will have times where you ask yourself "I at least did half of what i am suppose to do so I am fine with that". But you never will have that full satisfaction until you completed what you wanted to do in the first place. So, I got it done and Jen made my happiness touch the sky when i opened the door to breakfast all ready and even more- a huge smile on her face.
Swim - at lunch time, I was tired today. But after talking to Jen ( it really helps to talk to someone if your lacking the motivation) I decided to drive to the pool and see if I woke up once I Jumped in the pool. Funny, sometimes it takes a good jump of faith to wake up ;)
Anyway, I dominated the 2.5mi swim and felt very accomplished.
These Blues dont happen often, but when they do- call a friend, think of the aftermath, talk to God, do whatever to assure you complete the task at hand.