you have three main sources of energy in your body : fat storage, glycogene in liver, and glycogene in muscles.
Glycogene is a glucose polymere; like packed sugar.
When you do mild exercice, most of the energy is taken from glycogene, then the glycogene is recompleted.
The rest of energy is taken out of fat degradation, how much depend mostly on genetic.
When you do long and slow workout, you consume more an more glycogene, and start to depleate reserves, so that your body need to use more and more energy out of fat. Once the workout over, your body will have to rebuild glycogene storage, and in the meantime, will keep using fat to sustain you.
Heart rate too high mean you spend more energy that you can burn using the oxygene you breath (there is a limit to how much oxygene per minute your can breath). In that case, your body create an oxigene 'debt', producing lactate acid in your muscles. When lactate acid concentration increase too much, your muscles start to work poorly, you burn, cramp etc... and at one point have to stop.
When you keep your heart rate ok, you burn can burn as much energy as you want without having the lactate bareer to stop you. You can consider it takes one hour and half on average to deplete your glycogene reserves, it easy to know when you reach that point : suddently, you have to slow down to keep going. As fat burning gives you less energy by minute than glycogene, it becomes the limiting factor, instead of oxigene intake.
Conclusion : if you do SHORT cardio workout, you can make it intense if you want, you will spend a bit more energy, but you will also have more tired and sore muscles. Not to say it's not healthy to stress you heart too much if you are not in perfect heart condition.
If you are big on weight loss, make instead long cardio session, with comforatble hearth rate, at least one hour, you would be able to burn more fat than with a short intense workout, and, additional benefit, you will increase your heart fitness. If you do this regulary, you'll notice you heart rate speed at wakeup to slow down.