Hi, I've been doing steady cardio four times a week for a while now, and am thinking of adding an HIIT session per week just to spice up my workouts. I've started by alternating between 4mph and 8mph (doing 2 minutes on each) seven times. It usually adds up to less than 3 miles in total, so I continue doing steady cardio to complete the 5 miles I usually do. But I end up feeling really tired afterwards and my muscles are sore for the entire week. Should I be doing something different? My legs are sore after 30 minutes of interval training, but if I stop, I feel like I'm exercising less than I usually do (I'll be doing 18 miles a week instead of 20 miles). Should I include interval training as a separate session in addition to my steady cardio sessions (ie. do one session of interval training as well as 4 sessions of steady cardio per week?)