Wii Fit's aerobic activities have fixed course distances:
Distance is pre-set; your Mii's pace only affects completion time, not distance. You cannot adjust course length in the original Wii Fit. Wii Fit Plus adds "Rhythm Boxing" and "Obstacle Course" but keeps the same jogging/running distances.
For longer cardio, repeat the activity or combine with Step Aerobics, Hula Hoop, or Rhythm Boxing.