I am not that familiar with Exmarks so I cant tell you exactly how to adjust it. Most hydro drive machines have a provision for adjustment forward and reverse. The idea is that you limit how far the shaft going into the pump turns when you squeeze or release the hand controls. With the engine off have someone move the controls and watch the linkage move. You should be able to see an adjuster close to the pump. You always slow down the fast side because you cant speed up the slow side.
The Exmarks have a white star adjustment wheel located under
the seat between the pumps and can be rotated a 1/4 turn and then tested. My manual said that some of the older models
(goes by manuf. number) cannot be adusted and that you would
have to compensate as needed.