average cycling speed 12km/h
You can calculate your speed vs cadence using this online calculator. You want your cadence to be roughly 80 rpm (60 being the lower bound). 

With a 42t chainring and 14t cog in the back (not uncommon for a 26" mtb -- this is big ring in front, small in back), you should be doing around 31 kph at 80 RPM, so chances are you're either spinning too fast in a too low gear or spinning too slow in a too high gear.