Submitted by Scott Loughney:
From Kathmandu (Rangasala Dasharath Stadium) at 1,400 meters (4,593 ft) thru Jiri and Lukla to Mt Everest Base Camp, which sits at an altitude of 5,360 meters (17,585 ft) where the thin air contains only about 50% of the oxygen levels present at sea level and then back to Kathmandu (Rangasala Dasharath Stadium). Round trip distance of 640km (approx. 400 miles), with approximately 25,000 meters (82,020 ft) of brutal climbing and 25,000 meters (82,020ft ) of descending over the whole journey.
The overall Mailrun route is known for being extremely mentally and physically demanding due to its combination of high altitude and considerable variations in terrain.
Few people have attempted a one-way journey in a single effort and as far as we can tell, no one has ever attempted a round trip journey to establish a record. The fastest known time for the traditional one-way route was set by British ultra-runner Lizzy Hawker in 2013 on her 3rd running of the route in 63 hours and 8 minutes for the FKT.