too much running is not good for the knees because of cartilage
wear.
I used to run 4km every alternate night.
I have this neighbour, she's able to run 10km on the treadmill
everyday and she has a kid.
I was quite shocked by the distance, one day sat by the pool
with other neighbours to confirm
that she stays close to one hour on the treadmill and the
treadmill showed 11km.
Her heartbeat was abnormally slow, she even went to a
cardiologist for checkup.
Can't remember if it was around 50-55 bpm or something,
very very slow heart rate.
Nowadays I reduced running, and increased gym-ing...
Last few times i measured, my resting heartbeat varies between
57 to 62