[robot_jesus]

Well, the marathon record has been broken 53 times since the early 1900s. So, there are a lot of factors at play. Better training, better nutrition, better tactics, and, yes, better shoes.

The advancements in shoes have made a measurable impact, but there are lots of optimizations being worked on.

[tialaramex]

Also population and access. In the time since the early 1900s a lot more humans exist, and more of them have the opportunity to attempt this record. Population in Africa exploded in that time and access improved significantly.

If you're bloody quick and born in Birmingham (either of them) in 1900 you can probably find out about and get yourself a chance to attempt that world record, but if you're born in Kapsabet (in Kenya) in 1900 good luck, even in Nairobi I wouldn't bet on it.