You've probably heard the expression "running is more mental than physical." Anyone who calls themselves a runner will tell you that could not be truer.
Of course, it is your body doing the physical act of running. The more you run, the more your cardio improves, and the easier it gets physically. However, it's your mind that's going to allow you to push yourself that extra mile or miles. If you're just getting into more rigorous cardio exercise, it's not uncommon to have that little voice in your head saying, "this is awful" or "there is no way I can go another mile." The trick to overcoming that barrier can be as simple as thinking positively and ignoring negatives.
Instead of fixating on that part of your mind thinking negatively, begin pushing all those thoughts away. Start thinking, "keep going," "don't stop," "you've got this," "don't give up," or anything positive to keep yourself going. Keep reminding yourself that your body is more than capable of completing the run you're on.
It can be difficult at first, but the more you hang onto those positive thoughts, the easier your runs will get. Before you know it, you'll be jumping from having a difficult time running two or three miles to running five, six, seven miles almost easily. You will likely forget what it was like to have all those negative thoughts so present in your mind.
In fact, after a while it becomes almost natural to ignore all the negatives. You'll find that it's second nature for you to begin a run and never doubt that you are going to finish all the miles you set out to do. This trick is easier for some and difficult for others, but if you stick to running and are determined to push yourself to a new level, you would be shocked how something as simple as thinking positively can help.