If the rules of a 100 mile ultra walk state “no running”, then no running is allowed. If the rules of a 100 mile ultra RUN state “no walking”, then so be it, no walking should be allowed. But so far, I’ve yet to see a single race with a “no walking” rule. Pete Kostelnick and Johnny Hällneby has just finished a 6-day ultramarathon race at 3 Days at the Fair in 2018. Lotta Thörn is tending to Johnny, taking off his shoes and giving him a leg massage immediately. At this race, Johnny set a new Swedish National Record for 6-day running with 547 miles (880 km) and also won the competition. Saucony is a good one with 8mm or 4mm drops in most of their shoes but there are many more. With a lower drop shoe, you might be able to progress in 3 weeks to your normal distance but not in barefoot shoes. After walking in barefoot shoes for a few weeks, you can start doing 5-10min easy jogs.
I think that is the fastest any competitive ultra runner has done a marathon. This article says Goggins ran 3+ at the vegas marathon when he was really hurting. Don't know about Goggins, but Ian Sharman is a top ultrarunner and has a 2:21 marathon time. Some ultrarunners just don't do marathons, like Kilian Jornet.

It can hardly be the first time the Barkley has left an imprint on a runner’s brain. Camille Herron on her way to breaking multiple world records at the Jackpot Ultra Running Festival held

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can anyone run an ultramarathon