Wikipedia
Birkebeinerrennet (lit. The Birkebeiner race) is a long-distance cross-country ski marathon held annually in Norway. It debuted in 1932 and has been a part of Worldloppet as long as Worldloppet has been around.
The Birkebeinerrennet is one of three races held under the Birkebeiner moniker, the other two being Birkebeinerrittet (bicycling) and Birkebeinerløpet (cross-country running).
Researchers at the Manchester Metropolitan University found that the metabolic cost for modern ski models is about 2.5 J/kg*m as compared to about 5 J/kg*m for ski models dated 542 AD. They also estimated maximum cross country speed for the 542 AD ski to 5 m/s, compared to about 12 m/s for modern cross country skis; for long distance skiing (several hours) results were 3 and 7 m/s respectively. This corresponds to about 5 hours for the original 1206 AD Birkebeiner flight.