It always starts out the same. There you are, enthusiastic and happy, ready to meet the world and do your best or die trying. And then you do well. You sail. And then you coast. And then, ever so incrementally, it all comes to an imperceptable halt, about two thirds in, too close to done to really quit, too much to catch up to have a prayer. And you smile, and evade, and wonder what's going to derail you next round.
After a while, you stop trying at the things you know you'd love to do, and focus your efforts on the things you know you've got to get done. That way, when you come to that inevitable gentle halt, you can't blame yourself for going out and having the fun instead of doing the work, you have yourself to blame for just not doing it. Just not caring.
Just not trying, anymore.