HIIT shouldn't be done more than 3 days a week. Try alternating the two if you are the type of person who can do cardio daily without losing muscle mass.
Another idea: isntead of straight up and down intervals, try doing building intervals. For example, on a treadmill, I'll warm up (at 6.0 mph) for a few minutes and then do one minute of each of the following speeds (these are MY speeds, you may need to go faster/slower, so start at what is your "easy" pase that you feel like you could do for a long time): 6.5 mph, 7 mph, 7.5 mph, 8 mph...repeat for 20 min (5 times) and then cool down. It really helps build endurance and it's especially good if you have a quick recovery time.