One reason why many aspiring runners never actualise their running goals is they set themselves unrealistic running goals. Saying I want to finish a 42K marathon in under 3 hours when you have never finished a marathon before is setting yourself up for disappointment.
Set small goals that are concrete, attainable, realistic.
Decide to go three miles, regardless how many times you walk, how many you run. Or go out for 15 minutes. Or set one lap around the block as your starting goal.
Set a Time Limit
Set dates for achieving your running goals. Do your first 5K in six months; your first marathon next year. This will prevent you from the curse of procrastination.
Set an Incentive
Make running a means to a higher purpose. Is it losing weight? Fitting into your wedding dress? Running a Full Under 4 hours? Set a goal, and suddenly you have incentive.
Now got out and run!