[pm90]

My knees couldn't handle the added stress

This. I didn't know what HIIT was, but I figured out something like it for my daily exercise. Result was repeated shin-splits and painful knee injuries

[PeterisP]

You may want to try and find specific excercises to improve your joint-stabilizing muscles - even something very simple like http://archive.mensjournal.com/wp-content/uploads/picture-81... should allow your body to do higher intensities safely.