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duncs
You don't understand.
Yes there's enough energy to do it once, not repeatedly.
If you're on Earth, or in it's orbit, you can expend a large amount of thrust to start moving in 1 direction, but to move in completely the opposite direction requires an equal amount of thrust. So a Saturn V to get you going, but then another Saturn V when you want to go backwards. You could wait until the planets literally align and only do it when Mars and the Sun are in a straight line for the majority of the flight time.