I broke 5 this summer (4:57) with a 5K PR of 18:08 earlier this spring. I never have ran an all out 800 but a few times I ran 2:21s during workouts in the spring time. 35-45 miles per week typically.
It came at the end of a 1 mile race series 3 weeks long... first week was a PR, second week a tie, then under 5!
Ran 5:07 negative split 800 being conservative
ran 5:07 positive split 800 being too aggressive
4:57 negative split 800 and holding on for dear life during lap 3. making adjustments and learning from the first 2 races helped me the most.