Does running long distances hurt your body in the long run?

firstly, i hope u've spotted my mild attempt at a pun in the title.

but more importantly, i've visited a doctor recently who told me that running long distances hurts your cartilages (i run 5-10k a few times a week). He said walking is better for the heart and better for the legs, so when I am older I don't have problems.

He further explained that a runner's high is your body enduring the pain that your putting it through, so it produces endorphins but the pain is more destructive to the body than it is healthy.

Do you have any knowledge in this? what do you think? I love running and would hate to give it up.