If this http://www.hobbico.com/tools/hcar0454.html is the Hobbico mat you bought it is not meant to be used as a building surface. It is for cutting things. The blade cuts into the mat rather than a hard surface. Blades last longer and your hardsurfaces do not get scratched.

You can get a Balsa building board http://www.greatplanes.com/accys/gpmr6946.html that holds pins very well. It is expensive but I like mine. I have the 16x48 size.

Sheetrock is commonly used as a building surface. You'll want to tape the cut sides so it doesn't make chalky mess in your work area.

Acoustical ceiling tiles (smooth side up) can also be used.