I am about to try HIIT training for the first time. At the moment there is very little Cardio in my routine, and as much as I love lifting weights -it won't burn up my belly.

My question is regarding the running part itself, I was wondering if somebody could tell me if the following statement is true:

Is walking acceptable during the recovery stage? Or is this slack training? Considering I'm a novice and just starting out it would be favourable to me.

Sounds like very good advice to me. You might try walking for the first few times you do it and then try to jog very slowly later. Remember the most important part of HIIT is the HIGH Intensity Intervals, not the in between stuff. The HIGH Intensity Intervals must be 100% all out sprints, such that at the end of 15 seconds you can't possible keep up that pace any longer and after the 6th one your chest is burning and you feel like puking.

I know it varies per person, but what speed is considered a full sprint for a pretty skilled runner? 10 mph just isn't fast enough for me...

Speed is irrelavent as long as you're running as fast as possible flat out and pushing the pain barrier.

I'm not very fit.

Won't I have a heart attack?!

A full sprint is as fast as YOU can go. Go to a track and time yourself in a 100 yard dash. I think the average fit person should be able to do around 12-14 seconds, if you have long legs or are quicker than most then 10-12 seconds, world class sprinters do just under 10 seconds? Translate that to miles per hour: 1mi/5280ft*3ft/1yd*100yds/12sec*60sec/1min*60 min/1hr=17mi/hr?