The population of Essex is 1.7m. I doubt there is much beyond the Essex boundary that is less than 60 minutes drive to Southend at normal times.
I regularly drive around the M25 to Southend and can usually cover the distance from South Mimms Services to Southend in under an hour. So that brings a few million people in north, north east, east and south east London within 60 minutes of Southend. Mind you, most of them are closer to other airports.
Marketing will be the key - Southend does have the advantage of swift passage through the terminal and a rail station with a direct line to central London within a few metres of it.