When it comes to MMA conditioning training, there are obviously a lot of different factors to consider. You must train your aerobic conditioning so that you have the endurance and "wind" to go the distance in a fight. You must also train your anaerobic conditioning so that you have the endurance to continuously push the pace fast and explosively without slowing down to suck air. And you must train your muscular conditioning so that you have the strength, power, and endurance to exert the maximum output from your muscles throughout the fight. I have covered in more detail each one of these types of conditioning in other articles if you want to learn more about the difference of each type of MMA conditioning training.
Though it is important to train all three in harmony with each other in your MMA conditioning training, you must not forget one of the most basic principles when it comes to conditioning and endurance: consistency in your training. You see, in order to get the most out of your MMA conditioning training you must combine both quality AND quantity, in other words, you must train hard on a consistent basis.
However, I understand many MMA fighters, trainers, or just fans aren't necessarily fighting or competing on a regular basis, if at all. In this case, it is much more important to train your MMA conditioning consistently over a long period of time, even if you don't put in 100% in every MMA conditioning training workout. This is not an excuse by any means to slack off, but the point I am getting at is that your conditioning, like most things, cannot simply be improved overnight. It takes months of CONSISTENT conditioning to develop optimal levels.
So by knowing this, we can take a longer point of view in your MMA conditioning training. It is more important to go for a long distance slow paced jog on a day where you don't feel much like training then to skip it altogether because you don't think you have the energy that day to sprint 100% up steep inclines.
The cost of skipping a few days of any type of conditioning, even a slow and long distance aerobic jog, can really set your level of conditioning back. I have personally experienced this on several occasions where I was so busy that without me even realizing I hadn't done any type of MMA conditioning training for three or four days in a row, then when I went to train in Brazilian jujitsu the next day, I could barely make it through the 30 minute conditioning in the beginning of the class, let alone spar for the last hour.
So if you have been training your MMA conditioning hard for a while, just remember that it is always better to do some "light cardio" on days that you don't feel much like training then it is take off several days in a row doing nothing. It will pay off in the long run (no pun intended).