The mathematician is not alone in his passion for walking; younger people have also discovered hiking for themselves in Corona's time. One of them is Franzi Birker, 19, a maths student at Bonn University and a member of Stefan Hartmann's "school team" at HCM. "I told Stefan that I got into hiking at Lockdown. I was out and about a lot in the Ahr valley on weekends before the flood.“

Reason enough for Hartmann to invite Birker to the "Megamarsch", which takes place all over Germany in different cities and on a wide variety of routes. The goal is to walk 100 kilometres in 24 hours. "I like walking anyway, I've seen advertisements about it and videos of people trying it and breaking off. At the time, I thought I'd just give it a try - and then I did it twice in a row. In 2020, the march was unfortunately cancelled.“

Of all those Hartmann had invited to the march and who could imagine venturing together on the 100-kilometre route around Cologne, only Birker came along in the end. "Many people like the idea, but then cancel. That's a pity. It's really hard to find people who get involved.“

At the end of September, the 49-year-old and the student set off: at 12 noon at Fühlinger See. "Normally the route was nicer," says Hartmann: normally it goes from Brühl to Nettersheim. This year that was not possible because of the flood. So forests, parks and motorways, a lot of darkness instead of an Eifel idyll. "Running at night is a mental challenge, you can't do it at all without a headlamp.“

"At the beginning we talked a lot, I also listened to music in between. At the end, you don't have the energy to talk for long," reports Birker, who swapped her walking shoes for trainers after 40 kilometres ("much more pleasant!“).

She stopped running after 63 kilometres - "I just couldn't do it any more. Next time I'll take a smaller backpack," she says. While Birker waits to be picked up, Hartmann gathers new energy while waiting and makes it to the finish: "You're full of adrenaline and totally high, super awake, then you get tired all of a sudden. You shouldn't drive at that point," says Hartmann. "But I don't drive a car anyway.“

100 kilometres in 24 hours: If you walk just under five kilometres per hour, you can even do that with breaks. "You might ask yourself whether everyone should do it. No, not everyone has to do something so extreme. But everyone can ask themselves whether they really have to drive every distance. You can also get bread rolls on foot," says Hartmann. "It's just fun to walk, that's my message, but not with a raised forefinger.“

Birker is also still on fire: "The day after the march, I spontaneously went to the Moselle - to go hiking." She and Hartmann are planning a 50-kilometre march next, and Hartmann is tempted by an ultra march: "That's 171 kilometres. I could well have gone further.“

But Hartmann, a mathematician, is not after best times: "I think it's great to be in nature, you have time to think, you are completely with yourself. I like this purist thing. You just go for it. It's like mathematics, you can do it anywhere, anytime, anywhere in the world, no matter who you are or how old you are. Things like that, where you don't need much equipment and you can just start, that's what I like." (Original text: Maike Walbroel / Translation: Mareike Graepel)