I've been invited to hike the complete Pacific Crest Trail (PCT) with a friend. It seems like an impossible trail length of 4200km or 2600 mile that I'm doomed to fail on. But I know it could be done. What are the motivations to complete the PCT?
The main motivation for most people will be that they have done it, that they completed something that seemed very hard to impossible.
If that is not enough to motivate you, you will have to look for things that motivate you, personally.
That can be that you reward yourself when, and only when you have finished. Maybe you can find other rewards. It can also be the satisfaction of having helped your friend to accomplish the trail.
On the other hand, it can be that there is no way to motivate you other than starting and going on, hoping you get the hunger to finish while on the way.