Karl Meltzer's 46-day ultramarathon record

He stands at the foot of Maine's Mount Katahdin. He is 3,524 kilometers away from making history.

Forty five days, 22 hours, and 38 minutes later, American ultrarunner Karl Meltzer, with strapping on both knees and a head torch to light the way, arrives at Springer Mountain in Georgia.

He slaps the landmark sign in delight. No wonder -- he's just completed the equivalent of going up the Empire State Building 443 times.

Since August 3, he's been running an average of 14.8 hours a day, burning 345,122 calories, crossing 14 states, and working through 20 pairs of running shoes.

The Appalachian Trail runs along the east coast of America and is a favored route for ramblers. It mainly covers a terrain of forest trails and open fields, but also traverses roads, towns and farms, taking in some breathtaking scenery.

The Trail would take hikers about six months to complete in its entirety, but Meltzer's record took just a quarter of that, though his record attempt has been nearly a decade in the making.

"I've been trying to do it for nine years and the Appalachian Trail has been one of my favorite trails ever to run on," he told CNN. "Now, after trying three times, I finally get the chance to say I'm the king of the AT."

The previous record of 46 days, eight hours and seven minutes was held by Scott Jurek, a friend, training partner, and rival of Meltzer. Jurek ran his record from south to north, the opposite way to his counterpart.

"It was great to beat Scott Jurek's record, too," said Meltzer, "and to have him there at the end to run with me. It was really an honor and inspiration. Now it's my turn to hand over the baton to someone else."

After one and a half months of four o'clock starts in the morning in order to run 50 miles, Meltzer's feat was grueling and, in his own words, miserable.

But the 48-year-old is by no means alone in his quest to carry his body over immense distances.

An ultramarathon refers to any race longer than 42.195 km (26.219 miles), the length of a regular marathon. They can take place over a succession of days, or simply involve a single race, usually measuring between 50 and 200 km.

These races are often made worse by punishing conditions overhead and underfoot. The Marathon des Sables (MDS), for example, is a six-day, 251-km race across the Sahara Desert. Not only is the soft sand notoriously difficult to run on, but temperatures also often exceed 120°F (50°C).

It's not for the fainthearted. Blisters can develop from the smallest grain of sand getting in your shoes, and feet can swell from the heat. There are also more long-term problems:he has suggested that running such extreme distances may cause damage to heart tissue.