Original: His estimated distance between the cities was 5000 stadia (about 500 geographical miles or 800 km) by estimating the time that he had taken to travel from Syene to Alexandria by camel.
Simplified: His estimated distance between the cities was 5000 stadia (about 500 geographical miles or 800km) by estimating the time that he had taken to travel from Syene to Alexandria by camel.