Old computer desks tend to wear pretty quickly, but I've found even laminate desks can be coated with poly, I just roughed up the desk, used polyshade black to cover over the pattern on the desk entirely. After a couple coats the desk is pretty uniform, and I just masked off a rectangular section and coated that part with satin polyurethane. I did a few coats and my tape job wasn't perfect, so the edge isn't perfect, but its good enough, and it creates a surface your mouse can track, and one where theres no edge to catch your wrist on, furthermore you can have as much mousepad as you want, or you can simply coat your entire desk with satin and be done with it.
The only downside of this method is that poly has some pretty bad fumes for a few days, so leave the room ventilated or simply leave the desk outside. An oil based paint as first coat would also work. It takes about a week or two before the poly cures hard enough to use as a hard wearing mouse pad. During that time you can just use a regular pad on top.
Polyshade or oil based paint
oil based satin polyurethane