1,388 miles between Moscow and Athens. Not sure what the fuel tank capacity is for the planes which crashed/disappeared.
TU-134 range: 2,800 km
http://www.aeroflot.org/tu_154.htm
TU-154 range: 4,000 km
1388 miles = 2088 km
And they were headed south, towards Greece.
About 400 miles from Moscow to Athens on my map.