[ChrisInEdmonton]

I'm quite capable of running a half-marathon and decided to try the HIITs. They were terrible for me. My knees couldn't handle the added stress, and I had to give up HIIT and go back to 'regular' training. I'll certainly grant that it was an interesting training technique, though, and I can well see that it would be good for most people.

[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.