One of the hardest parts about training is getting and staying motivated. The trick for each of us is to identify what motivates us individually. Some run for charity, some to prove something to themselves, others to collect racing shirts and medals. Since no two people are the same, I am simply going to explain some of the things that motivate me, and maybe some of them will resonate with you.
The number one motivator for me is Race Day! Usually, I look out on the local event schedule and find a race that I want to run and I mark my calendar. I start to train for a few weeks to make sure that everything is in working order, then I register for the race and spend my money. At that point, I am committed. There is no backing out now. All of my training from that point forward is focused on Race Day.