Hey curlygirl!
Recommended effort for best results (i.e. fat loss & cardio improvement) is 65-75% of maximum heart rate.
Max heart rate is 220-age so yours is 191.
65% of 191 is 124 and 75% of 191 is 143. So you should be aiming for a heart rate of between 124 and 143 beats per minute. Ideally - keep going for 45 mins for best results.
Good luck
xx