This familiar equation E=MC2 is actually a special case of the equation Einstein discovered. It is the special case where the object in question (which we wish to calculate the total energy available) is at rest - that is, its velocity is "0". The full equation is:
Energy = MC2/Sqrt ( 1 - v2/c2)
The last part of the equation tells us that the Energy of an object increases if the object is traveling a velocity of v.
In particular, if the object in question is standing still, (v=0) the equation reduces to the familiar E=mc2 .The reduced equation is not true if "you" are not standing 'still' relative to the object in question!