How can I trim my pegboard?
Use the circular saw.  

Clamp a piece of scrap 1" by something down to guide the saw for a straight cut. This will take some time to lay out, but it will be worth it.

Get your saw up to speed before cutting the hardboard.

Additional Tips from the comments:

Put masking/painters tape on the cut line and cut down the middle of the tape.  This will reduce splintering of the edges.

If using a circular saw, cut with the finished side down.  Splintering, if any will occur on the back side, which is hidden.

Pro-tip: A table saw with the blade lowered makes a perfectly serviceable dining room table.  It also makes carving a roast a breeze.