HIIT And Bad Knees?

I have bad knees, have done since I was about 13, and so can't run. I also think running on hard surfaces is bad for your joints anyway so I just power walk everywhere. I've really noticed an improvement in m fitness levels - I can now do a walk in about 25 mins that I used to do in 40.

I've been thinking lately that the only way to improve my fitness even more would be to start running and my knees have been better since I've invested in sme trainers and insert things to support my arches. So I was wondering about HIIT. Since you only sprint for like 20-30 secs at a time would this be ok for me to do?

Also, how do you log it in the activity log? Not that this matters hugely but would be interested in how many more calories I was burning than by just walking.

If I try it and my knees start giving up then is walking on it's own ok? (I do other excercise as well but I mean just when I'm outside)

HIIT doesn't have to be sprinting/running. You can do cycle HIIT [easier on the knees], jumping jacks, jump rope....etc. 

I'm currently recovering from a knee problem, and ended up giving up running too.  I'm sure I could go back to it, but I'd rather save myself for other stuff. 

I think jumping jacks and jump rope would probably be just as rough on the ol' knees as running, but the suggestion to use a cycle - either a bicycle or an exercise bike is a really good one. 

Thanks for your replies. I haven't got a bike but will try it at the gym. I'm going to give sprinting/walking a go and if it hurts I'll stop!