It might be because your warm-up is too intense? You want to start off pretty slow and increase the pace. If you try go from 0 to 70 or 80 suddenly, your heart and body are having to work awfully hard. Once you are warmed up, you can try going at 80~100..
Swimming is a great exercise but uses muscles in different way from running or tennis, in other words, it's not a great exercise to prep you for tennis. If I were you, I would substitute stationery bike and treadmill with one of swimming sessions. Try interval training, which is fairly easy to simulate on stationary bikes.
Pvaudio's suggestion is interesting. One of the reasons why boxers can last so long throwing punches is because they know how to breathe, which is quite similar to tennis. Whenever you hit a shot, try exhaling. You inhale as you wind up and exhale as you execute - you see pros do this all the time. If you find this unfamiliar, simply trying soft grunting. It helps me time and recover.
You might also have too high of expectation from your playing days (I assume you played college tennis??). You are probably mid to late 30s, and are not playing tennis on daily basis.