If you admit to have only one international airport :
AKL-OZP : 12413 mi / 19977 km
Auckland [Intl], NZ -> Seville - Moron de la Frontera [Moron AB], ES (11801 ft rwy)
http://gc.kls2.com/cgi-bin/gc?PATH=A...&PATH-UNITS=mi
or even better :
SYX-ARI : 12430 mi / 20004 km
Sanya [Phoenix Intl], CN -> Arica [Chacalluta], CL
http://gc.kls2.com/cgi-bin/gc?PATH=S...&PATH-UNITS=mi
longest found (neither international) :
SFD-MLG : 12433 mi / 20008 km
San Fernando de Apu, VE -> Malang [Abdul Rachman Saleh], ID
http://gc.kls2.com/cgi-bin/gc?PATH=S...&PATH-UNITS=mi
CCK-RNI : 12438 mi / 20017 km (knowing that the theoretical maximum is : 12450mi / 20037 km)
Cocos (Keeling) Islands, CC -> Corn Island, NI
http://gc.kls2.com/cgi-bin/gc?PATH=C...&PATH-UNITS=mi
[edited 'cause one just found an even longer one]
BXE-ZGU : 12439 mi / 20019 km
Bakel, SN -> Gaua, VU
http://gc.kls2.com/cgi-bin/gc?PATH=B...&PATH-UNITS=mi