When your motivation gets low, sit down & make a list of everything you are gaining by continuing to work towards your goal. And make it personal: Great health; no medical bills; can play w/the kids; look great; run w/o getting winded; get a date; dance; make friends jealous, etc.
Be sure to hang it where you can read it every day. THE REWARDS ARE ENDLESS AND WORTH EVERY BIT OF EFFORT!!