One of the hardest aspects of jiu-jitsu is being conditioned well enough to execute the necessary techniques you are trying to use. You see a lot of people come into the gym that say they are in very good shape, but once they get onto the mats, they quickly realize that is not the case. Just because you can do one sport or activity efficiently, it doesn’t mean it is going to cross over to the next.
It is beneficial for any athlete to cross train no matter what sport they do. This is why you see a lot of athletes doing a variety of things from swimming, running, strength training, and so on. It is important to work different energy systems to be well rounded. Jiu-jitsu is a very unique sport where you are basically using two different systems simultaneously (aerobic and anaerobic), so you need to be working both of these if you want to last multiple round in a row.
You see too many people taking round off because they do not know how to balance between the two. Most of them are using their anaerobic system too much and they deplete all of their energy doing this. Individuals try way too hard when attempting a submission they are probably not going to get while the other person waits out until they get tired, and then you know what happens after that. When they are doing this, it is usually in unnecessary situations, so it is wasted energy. The need to learn when you should or should not try a move will help greatly in conserving energy.
This is why you should focus on pacing yourself while you roll in most cases. If you are training for a competition that has shorter time limits, this will change the way you approach training. Also, depending on the rule set, this will change the methods you use to get properly conditioned. As more and more tournaments are going to the sub-only format, pacing yourself while rolling will be even more important.
Work on being patient when attempting any moves, and be smart with the energy you are using. You would rather get more quality rolling sessions in where you are using good technique rather than one or two where you drain all of your energy. Another perk of this type of training is that you will be able to train more days per week. Remember, consistency is key! Train hard but train smart!