On Hard you'll be hard pressed to keep up even if you race a clean lap. Overall the medium setting seemed to put you on par with the computer racers. In practice mode you select a race and compete against the clock only. And in championship you compete over each course in turn. To advance to the next course you must finish in the top three for the current course.
Courses alternate between super bikes and motocross.
I found the faster pace of the super bikes more interesting than the motocross. The super bikes were also easier to control, at least for me. I had a hard time getting the hang of turning quickly on the motocross circuits. A lot there may depend on your controller too. A joystick worked best with the super bikes, whereas my game pad seemed best suited to the motocross.