I agree with GamingGrrl.
It's your life, not his. You enjoy riding, stand up for what you enjoy.
Both riding AND running are hard on your knees. My knees at 24 are trashed from years and years of riding. Doesn't help a four year old mare broke my knee a couple years ago, but it can be strenuous on your body.
'Course I'm a thick rider, so I'm sure between walking with my weight and riding, hastens the trauma to my knees. Regardless, both will wear/tear down your knees eventually.
The Fluffy Equestrian
Rhett | Hang on "Fiona" | Rockstar
Reg. TWH Gelding | Irish Draught Horse Mare | SSH Gelding