I first have to say that I am not a personal trainer, or a doctor and that I am not telling you that you have to do this exercise routine. Please consult a doctor before any exercise.
There are two things that have built my endurance to run faster and work out longer. First, the bleachers. I run them as fast as I can and I run them back to back. I have increased the number of how many sets I do over time. Obviously be careful. Second, my run walk sequence. My normal 5K run I split up into run walks. I will run the length of a block faster than I normally run. Then the next block I walk. I have increased it to two blocks but I won't do more than that. Both of these help me control my breathing at my fastest and gets me to realize my potential. Sounds funny but it is very true. My husband does this with me and it has helped him as well. Even if you are not a runner this will help you in any work out routine. This also gives you a different work out. We all know that is crucial when it comes to staying motivated. Good luck and have fun!