Just in case anyone is interested here are some stats on runway lengths I found.

Area 51's Runway of 7481m is not in use anymore. You can tell by the large crosses and bad state of repair of the runway.

Upington is the longest paved runway in the world.

For comparison:

Denver International Airport:
Paved Runway length: 4877m

Edwards Airforce Base:
Unpaved runway length: 12.1km
Paved Runway length: 4576m

 The following is from Wikipedia . According to them this isn't the longest in the world. It is the longest in Africa at 16,076 ft (4900 m) long.

Longest Runways

Some of the longest runways include:

Qamdo Bamda Airport, Tibet, China (30°33'N 97°06'E) - 18,045 ft (5500 m) concrete, of which 13,779 ft (4200 m) currently meets 4D standard. Bamda's runway is also the highest in world, with altitude at 14,219 ft (4334 m). [1] [2]
Ulyanovsk-Vostochny International Airport, Ulyanovsk, Russia (54°24'N 48°48'E) - 16,404 ft (5000 m) concrete [3] [4]
Shigatse Peace Airport, Tibet, China (29°21'N 89°19'E) - 16,404 ft (5000 m) concrete, not in use, renovation starts in 2007 with projected completion in 2009. [5]
Embraer Gaviao Peixoto, Brazil (21°46'S 48°24'W) - 16,295 ft (4967 m) asphalt [6]
Upington, South Africa (28°24'S 21°16'E) - 16,076 ft (4900 m) asphalt [7]

Cheers, DW