Max Jones and George Black have got hold of some tables published by the World Association of Veteran Athletes that enable you to compare performances at different distances and/or at different ages. For example, which is the best performance?:
10k in 35:41 aged 40
Half marathon in 1:18:45 aged 40
l0miles in 60:56 aged 45
10k in 38:12 aged 50
Marathon in 3:01:46 aged 55
Well, according to the tables, they’re all the same - all equivalent to 80% of the theoretical age best at each distance/age.