Not sure what level of exercise intensity you should do? According to the ACSM Guide to Exercise Testing and Prescription, 6th Ed., 2000 these are the guidelines:
If your purpose is to train to promote fitness – High Intensity Exercise
30 to 45 minutes 3 to 5 days per week.
Training Heart Rate: 70% – 90% of HR Max (60%-85% of HRR)
Rate of Perceived Exertion: 14-16
If your purpose is to exercise to promote health – Moderate Intensity Exercise
Accumulate 30 minutes most days per week.
Training Heart Rate: 50%-70% of HR Max (40%-60% of HRR)
Rate of Perceived Exertion: 11-13
If your purpose is activity to maintain function or prevent deconditioning – Low Intensity Exercise
3 to 5 minutes of activity that is well-tolerated several times per day at or below 50% of HR max,
gradually increase exercise time and intensity.