If you find working out to be a challenge, then chances are you think the idea behind HIIT is scary and you have nightmares about Shawn T. yelling "KEEP GOING!" in your face. However, HIIT does not have to be scary, and if done correctly, it should not be too hard either. If you find it too hard, then you're not doing it right. People often ask, what is more effective, endurance or HIIT? This is a bunk question because it assumes you have to chose one or the other, and it shows many people do not understand HIIT.
HIIT is basically taking advantage of the biological fact that you can catch your breath. If you're out of shape, you might not even be able to run a mile, but you can run 20 feet and stop and catch your breath... then run 20 feet again. The concept that makes HIIT work is dancing across the aerobic-anaerobic line and back again, over and over. This line is in a different place for everyone, so if it is too hard, then you've miscalculated where that line is or are making major swings back and forth across it instead of a small step up and then down.
Here is an example of how I integrate HIIT into my workout: my workout is designed to simulate rucking (walking or hiking with a very heavy pack) at 4 mph on a treadmill. I can adjust the weight I'm carrying anywhere from 0 to 150 pounds and increase the treadmill incline until I hover nicely around a 65% heartrate, at which I am still breathing normally. I can do this for hours without needing to stop or slow down... this sustainable starting point is the key to effective HIIT. You want to start with a baseline that you can do for very long periods of time without breathing hard (ie, without going over the line from aerobic to anaerobic). You should be able to, with little discomfort, talk to someone next to you.
Then, when I'm hovering right at or below that "infinitely" sustainable pace, I ratchet up one factor or another to put my body in a circumstance which it cannot sustain for more than a few minutes at most. For my workout, I use incline and for any sort of treadmill activity I find this works very nicely because the treadmill, not you, introduces this element, which allows you to maintain your stride which lends nicely to the solid baseline we covered in the previous paragraph. I ratchet up the incline a few notches... not much, because I'm already at the line so anything more should eventually exhaust me. Then, I stay at an incline until I feel like I need to stop or slow down, and I lower the incline back to my baseline value. You do not want to go so far that you are not even able to sustain your baseline value afterwards, as that would defeat the purpose. Your vitals will eventually drop back to what they were before, and you can do it again.
The reason HIIT works and is so effective is because now you have increased the intensity of my already indefinitely sustainable activity without losing it's sustainability, all by taking advantage of the fact that you can catch your breath. This means more cardiovascular development, calories burned, whatever you're going for. You can enjoy the endurance growth or fat-burn zone benefits during your long cool-down periods as well as the strength growth or fat afterburn effect from the shorter, high-intensity periods.