The problem with this mower is the pedal is very hard to push down to move forward or backward. I have to pull the pedal with my hand while using my foot to go in reverse. It does not go very fast when going forward.

Hi,
I find something wrong with your model number; There are only an X89A(1999) and X89B(2000) that were made. Here is a link to the latter:
http://www.outdoordistributors.com/pdf/Murray/MURRAY-MODEL-52161X89B-GARDEN-TRACTOR-(2000)-PARTS-LIST.pdf
and the linkage in question is on page 13. Something is bent and/or binding.
Peace,
Clarke