Set short term goals
By having small goals along the way you keep yourself motivated and inspired and that final goal doesn't seem that far away after all
If my run isn't going so well I will change my route so I'm making more turns or direction changes. That way I'm not discouraged by distance. I run "just to the corner" then "just to what ever landmark" I see. Soon enough my run is done and I did the full time I set out to do.