One thing that I used to motivate myself when I was coming back at your age was to use VDOT and age grade calculators to see how close to my old college fitness I could get.
For example, let's say that you were a 4:00 1500m runner in college. That means that your VDOT was about 70, which rates out to a 14:53 5000m run. But since you were more 1500 oriented, let's round that up to 15:15 or so, just for argument's sake. Based on the age grade calculators, that 15:15 as a collegiate would be the equivalent of a 16:10 at 40 years old. 16:10 (or whatever the equivalent may be based on your actual times instead of my hypothetical times) may seem out of range, but you could use it as a long term goal (2 years from now) and set some intermediate goals of 16:50 this year, 16:30 next year, and 16:10 the year after.
Admittedly a bit hokey, but it is an option.
I am doing the same thing, only with my HS times. No way at 42 I can reach my university times, even age graded.