I think I agree that you may just be cut out for shorter distances. I am a 59yo female and it has been about 15 years since I've been able to break 20 min for a 5K. I currently run in the low 23's for 5K, but this spring I ran three HMs in the 1:43-1:44 range.
Mile 6:11 (current fitness is 6:35 as I did a time trail the other day)
1.5 mile 10:00
From another perspective, in my early 30s I could run a 5K in just over 18 min; however, the best I could do for 400m was about 1:17.
You have plenty of natural speed, but you're going to really have to work on your endurance to run well at a distance as "long" as 5K.