How close is the sub two-hour marathon?

Andy Jones, Professor of Applied Physiology in the Department of Public Health and Sport Sciences at the University of Exeter, has worked with Nike as part of their Breaking 2 marathon project and Faith Kipyegon's recent attempt at breaking the four-minute mile.

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We got his thoughts on when, and how, this major landmark might fall.How close do you think we are to the two-hour marathon mark being broken legally?

I think it would already have happened if Kelvin Kiptum were still with us. I also think that Eliud Kipchoge would have done it if he'd run a marathon in the right place in the right conditions around the time he ran 1:59:40. I think it will definitely happen within 10 years, will probably happen within five years and could possibly happen within two years.

What do you see as being the biggest obstacle that is standing in the way? That is, which areas do you see as having the most room for improvement, given the rapid advances in technology, nutrition and so on?