Can you explain this exercise experience?

I haven’t been to the gym for a month because I normally go with my husband and he has been having back pain. Instead, I’ve been doing 5 days a week of at-home workouts. I usually do HIIT (high intensity interval training) that leaves me gasping for breath and sweating like a pig. I can do a 45-minute video and, afterwards, feel fine. I’ve also been doing some strength training/toning workouts.

On my first day back at the gym, I did about 20 minutes of lower-body workouts on their leg machines and then hopped on the treadmill for a 30-minute rolling hills walk at 4.0mph. I burned about ⅔ the calories I do when I exercise at home, and yet I was completely miserable about halfway through and felt like crap after I was done.

Now, this is just a brisk walk, not jumping jacks and burpees and all of those full-body movements I do with HIIT. It wasn’t even running. Why did it take so much more out of me, and yet burn fewer calories, than what I do at home (for free, I might add)?