In 2011, Rundgreen was Junior World Champion with the Norwegian relay team in Otepää and achieved tenth place in the skiathlon at the U23 World Championships in 2012 and 2013 . In January 2012 he started in Åsarna for the first time in the Scandinavian Cup and finished 175th in the sprint. In the 2013/14 season he came on the podium for the first time with two third places in the Scandinavian Cup and finished eighth in the overall standings. At the U23 World Championships in 2014 in Val di Fiemme , he finished fourth in the skiathlon and won the bronze medal over 15 km in classic style. In March 2014 he started the 50 km mass start race in the classic technique in Oslo for the first time in the World Cup and finished 33rd there. During his next World Cup appearance in Rybinsk, Russia in January 2015 he achieved world cup points for the first time with a 27th place over 15 km freestyle also finished the skiathlon two days later with 24th place in the points. In the Scandinavian Cup he took second place twice in the 2014/15 season and took his first victory in the Scandinavian Cup over 15 km freestyle in Lillehammer and thus came second in the overall standings. At the beginning of the 2015/16 season he achieved his first podium finish in the World Cup with second place with the relay in Lillehammer. This was followed by a third place in the Scandinavian Cup over 15 km freestyle in Vuokatti and a second place over 15 km freestyle in Östersund . In January 2016 he won his first World Cup victory with the relay in Nové Město and took third place over 15 km classic at the Norwegian Championships in Tromsø . At the end of the season he took eighth place in the overall ranking of the Scandinavian Cup. After finishing 40th in the World Cup mini tour in Lillehammer at the beginning of the 2016/17 season, he finished in the top ten five times in the Scandinavian Cup, including two third places and overall victory in the mini tour in Madona . In February 2017 in Pyeongchang he achieved his first podium finish in the individual World Cup with third place in the skiathlon. He finished the season in 54th place in the overall World Cup, in 34th place in the Distance World Cup and in third place in the overall ranking of the Scandinavian Cup.
Norway
Nové Město
Madona