Larsen akava shasha yenyika yeDenmark pamakiromita 5 000 ne10 000 muna 1937, achiwana mukombe wokutanga wekumhanya weHelsingør IF. Akaenderera mberi nekutora mamwe makwikwi gumi emunyika, mapfumbamwe mumarathon (1941, 1943<unk>1950) uye imwe mu20 km (1950), pamwe nekuuya panzvimbo yechitatu mu10,000m muna 1938. Muna 1946 akaisa rekodhi yenyika yeDenmark yemarathon pa2:38:36 uye akakwikwidza muEuropean Athletics Championships yegore iroro, achiisa yechisere mumarathon. Kumira kwake kukuru kwenyika yose kwakatevera kwakanga kuri Mitambo yeOlympic yezhizha ya1948, apo akauya mugumi mumunda wevakwikwidzi 41 mumutambo mumwe chetewo
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