But marathon and Half-marathon is not only about cardiovasucular fitness. It is also about invulnarability of muscles, kness, ankle etc
They need to be adapted. It is not a good idea
to extropolate your half-marathon and marathon
capabilities based upon 5 mile run pace
if you have not run any distance longer than
5 miles.

Steven is also very fast in 4 mile race but
he once had trouble even running longer than that