Hey there! I've been starting to do the HIIT workout maxing at 8.0mph. And I love it! But...it is really wiping me out! I usually run 4-5 days a week (usually weekdays, around 4 miles each time) and lift weights every other day, crunches every day. But with HIIT I fell like only running every other day, more like 3-4 days a week. It's not affecting the rest of my workout. I haven't gained (or lost) any weight - very close to my goal weight. Is this normal? How often are you supposed to do HIIT? Thanks for any help, and thanks for explaining HIIT to me. Hubby has started doing it, too.