I noticed during today's run that the hardest part wasn't the end. The final five miles probably hurt more than the other fifteen, but there's an extra emotional edge which carries you through. (After all, if you've already ran sixteen miles, it's not that hard to convince yourself to run another four.) The toughest part, the part during which I almost quit, was mile 2. While the first mile usually feels surprisingly good, the second mile is when your body starts to argue with you. "Are we seriously doing this? I was fine to jog down the street but you want to run for how long? I really don't think this is a good idea..." My legs seemed to speak to me as passed the first mile marker that I should probably just forget the whole thing. My subconscious seemed to whisper that the first mile was plenty, and there's many more days before November 22; I should just turn around and go home. It's the transition from "stationary person who happens to be running" to "running person" that's the most uncomfortable and requires the most mental energy to push through. It's really really easy to quit early on because you're a) not that far from the car and b) can tell yourself "ok, I'll just try again tomorrow". While you might be more tired later in the run, it's much harder to quit at that point because of all the work that's already been put in.
In many ways, this process describes the growing pains I see in many of my students. For group projects, they start strong and taper off as time goes by, but for the individual work, it's the first "mile" or two that's tough. Once they finally tell themselves that they need to switch into "work" mode and get something done, they're good to go, but for many, there's always an excuse to try again "another day". Aside from practice and some discipline, I'm not sure there's a quick solution. Things like to remain as they are. Success only occurs when things are able to change. If you're going to succeed, you have to fight your body and brain's desire to stay put. Don't worry, it gets easier once you push through and your subconscious joins your team and stops fighting you.