By far the hardest thing to overcome for any long journey is idea that it IS a long journey. Who the hell in their right mind says to themselves "You know, I fully understand this is going to be a long journey and I SO look forward to every day of it". It just doesn't happen.
Be honest. If you REALLY knew how long your life was going to be you probably would have taken MUCH better care of yourself, AND, you would have undoubtedly crapped your pants when first reading that lifespan map and said to yourself "THAT is amazing, and overwhelming all in one!"
Get this:
On the average, a person takes 3-4,000 steps a day, which rounds out to 1.5-2 miles day. Let's round up. 2 miles a day x 7 days a week = 14 miles a week.
14 miles a week x 52 weeks a year = 728 miles a year.
728 miles x 78.6 years (average life span of an American) = 57,220.8 miles in your lifetime.
57,220.8 miles in your lifetime/26.2 miles in a marathon = 2,184.
You are going to be doing 2,184 marathons in your lifetime there pally.
I'd recommend you get another pair of sneakers.