i really want to improve my condition and thus strengthen my heart. i thought of fast jogging for a long period of time (40 mins) would do the trick, or mountain-biking (coz i live kind of in the mountains) but i heard that these two activities are rather inconvenient when doing them often, coz they really put a lot of pressure on the heart and blood-vessels which can be pretty unhealthy.
any suggestions?
If you gradualy increase the amount of cardio you do it strengthens your heart. Start out with 15 minutes of walk/jog/run. Keep to that program for a week, next week add two minutes, next week another two minutes.
If you take your pulse for a full minute before you get out of bed in the morning, you have your resting heart rate. Take it once a week. Look to see if it is goping uip (overtraining) going down (heart getting stronger) or staying the same (not enough challenge).
Don't overexert - your heart will fatigue instead of growing stronger.