I don't need to guess.
 http://www.androcles01.pwp.blueyonder.co.uk/MC2.htm
Energy is very well defined in Newtonian Mechanics, it
is the integral of momentum with respect to time and momentum
is defined as mv.

What is not well-defined is the stupid assertion
  'we establish by definition that the "time" required by light to travel 
from A to B equals the "time" it requires to travel from B to A'.

Of course E = mc^2.
It doesn't mean E = mc^2/sqrt(1-v^2/c^2)