Recently I pulled together an interesting (to me) chart of my personal best road race record times at various standard distances, with a column for pace in minutes/mile and with the second-best results also included:
Eyeballing it, clearly I've badly neglected 5 miles over the years and have only run 8 km (= ~4.95 miles) seriously one time. Graphing the pace values versus distance on a log scale is even more revealing:
Half a dozen of the dots form a nice almost-straight line of records from 1 mile through 10 km. On the average, every time the distance doubles, my pace slows by about 0.3 min/mi. But those top three points are the anomalies, at 5 miles, the half marathon, and the marathon. Can they be pulled down to that same trend line, by more disciplined training, more mental toughness, etc.?