Quite right! There used to be a lever on the left hand handlebar that controlled the hydrostat. It sounds like your lever has been left in the reverse position so that when you engage drive to the gearbox it starts to move backwards. If you move the hydrostat lever to the middle position you will find it will stop the wheels rotating, and if you move it to the forward position it will move the wheels progressively faster in the forwards direction. The further forward you push the lever the faster forward it will go and if you bring it backwards it will get slower, then stop, then start to move in reverse.
Don’t run yourself over!