Have you tried an interval routine on the bike? Alternating the intensity so that your average heart rate is in the 55% - 75% range. IMHO you do not need to keep that rate sustained for the whole period. You have taken the first, and most crucial, step and that is to make a change. Try doing intervals of higher intensity and see how that works out. The idea is to put some stress on your body but not get into the extreme longer duration cardio routines.