I do heavy lifting in the effort to bulk up (natural skinny guy frame with fast metabolism – but heavily cut now), and am going to try this HIIT routine to get better definition (fat loss) a little faster than what I've been doing:
1. *****
2. Amp up the intensity on the treadmill so that performing an ALL OUT sprint for 30 seconds.
3. Decrease the intensity back to a fast walking pace. Walk for 90 seconds.
4. Repeat steps 10 times.
5. *****
My question relates to frequency of HIIT:

I currently do only what would be considered "moderate" cardio, in comparison to HIIT, but I do this after every workout, about 5 to 6 times per week. This is alternating between jogging (7 mph) and inclined speed walking (no sprinting) for 30 minutes.

If I adopt this HIIT workout, 3 times per week, should I abandon all other cardio, or can I do my alternative “regular” routine mentioned above on the days I do not do HIIT?

Could I end up "over training" and losing muscle mass by doing the "regular" Cardio routine outside of the HIIT 3 times per week?

Thanks for the advice!