I would tell you to just take the HRX and see how you feel before doing stuff like HIIT. Why?
I personally respond terribly to HOT-ROX Extreme. Max Strength HOT-ROX are fine, but HOT-ROX Extreme just don’t agree with me. Nausea, dizziness, percieved muscle weakness, rapid heart rate and insomnia. There’s no possible way I could do any sort of demanding exercise while taking them.
One pill per day will make me feel iffy. Two will make me feel ill enough to cut out all activity and stay home. I took four in a day one time, and felt like death warmed over. Felt like I had the flu.
Obviously, a lot of people take them and feel fine and get good results. Hopefully you will as well. But these are powerful little pills, and if your body just doesn’t like them, you’re just not going to be able to do much of anything.
So I would recommend you start out with one pill per day, preferably starting with a rest day. If you feel fine, go workout. Stay on one pill per day for a little while before upping to two per day or more.
Treat them with respect, because they are certainly powerful. And if your body responds poorly to them, all you’re doing is setting yourself back.