another data point wrote:
In college, I averaged about 40-45 mi/week w/ 2 workouts and ran 1:55/3:59 for 800/1500 + 25:40 8k in XC. Not great, but not bad either. In my mid-30s, I picked up running again and have run a 16:03 5k, 1:14 half marathon, and 2:48 marathon off 35-45 mi/week w/ 1-2 workouts. Obviously, my marathon suffers most at lower mileage, but 5k-half I think are doable. I'd like to run sub-16 and sub-2:40 before I turn 40 though, so I am planning to bump up to 50-60 next summer and see what I can do.
Very similar situation for me, although not nearly as fast in college (ran senior year only, 35-min 10,000). A year after graduation, I ran 16:04 and 2:40:30 off 42 miles per week.
Similar mileage and regular VO2max workouts netted 17:10 and 2:56:03 at age 43. Like in your case, the low MPW is most evident in the marathon.
At age 58, running 42 mpw average with dedicated VO2max and LT workouts, I ran 18:58 and 3:07:23. Again, the marathon is a bit weak by comparison.
Next year, at age 60, for the first time in my life, I plan to maintain 60 mpw, and aim for sub-3. Well, I should say, 60 mpw average will be the first time if I do it ... I have always planned to do it!
Good luck with your quest for sub-2:40!