The process of getting winded is actually quite complicated! There's more to it than just oxygenation.
Stanford developed a prototype to extend one's capacity for exercise. They didn't need an external cardiovascular system. All they needed was a fist. It turns out that a surprisingly large amount of what makes us fatigue is temperature. Our body decides to stop exerting itself if it thinks it can overheat to the point of damaging itself. They developed a heat exchanger that work with the palm of the hand, and it showed dramatic effects for increasing our endurance.
Oxygen actually has less to do with exercise than you might think. We're actually more concerned with getting CO2 out than oxygen in. CO2 levels in the blood make the blood more acidic, and that can cause serious issues for many metabolic processes. Meanwhile, hemoglobin is very good at its job. Generally speaking, even in extreme exercise, oxygen saturation doesn't dip below 95%. Below 90% is consider hypoxia, and below 55% is typically considered fatal (though there was some awesome data gathering done on Everest which showed climbers not only surviving at 40% saturation, but climbing. Just shows how much it's mind over matter!)
In fact, where you really run into issues with long distance running is energy. As it turns out, the brain is a bit of a conundrum for evolution. If you put enough energy storage (i.e. sugars) in the brain, it actually spreads things out too much and the brain doesn't do its job as well. The body has to store sugars for the brain elsewhere. The solution is marvelous, and centers around the hormone insulin. The brain actually subsists entirely on energy stored in the liver as glycogen (the animal equivalent of starch), and the liver releases that into the blood stream as glucose to be consumed. Of course, glucose feeds other parts of the body too, like muscle. If you were running too hard, your muscles might try to rob the brain of glucose, which could be bad.
The solution is insulin. All skeletal muscles and adipose tissue, which together make up 2/3 of the body's mass, are not permitted to pull glucose out of the blood unless there is insulin present. When you eat, your body recognizes that there's sugars/starches in the food, and releases insulin to permit the muscles to capture their share of it. When the food has been consumed, the body stops producing insulin, and the liver starts emitting the glucose it picked up during the feast. The only muscles that are allowed to pick up that glucose are the cardiac muscles and the smooth muscles which line hollow organs (essential for processing more food when it arrives... we also find smooth muscle lining the cardiovascular system to control blood pressure).
When doing long distance running such as marathons, runners experience what is known as the "bonk." It's a wall that occurs, for most people, around the 18 mile mark. What has actually happened is that the liver only stores about 4 hours of sugars to work with, and runners hit 18 miles around the 4 hour mark. At this point, your brain starts running into trouble. It literally lacks the fuel to keep making good decisions. The muscles still have plenty of glycogen to keep themselves going, but the brain has run out!
The solution is simple: a sports drink. The little bit of sugar in the drink quickly hits the blood stream and perks you right back up.
In the end, a suit may not be needed at all. Enter the world of Ultra-marathoners and the infoamous the iron man triathalon. Iron man is a 2.4 mile swim, 112 mile bike ride, and a 26.2 mile run, done over the course of a day. These individuals clearly can keep functioning for long runs, but there is a limit: sleep. At some point, you have to stop running to sleep. Any run which tries to avoid sleeping is going to have new problems that aren't solved by a mere cardivascular suit. For some sense of what those look like, consider looking at the demands of the military, where making it to the correct location is literally a matter of life and death. Even there, sleep is prized.
In the end, long distance endurance is a lot more than just oxygen content. There's a vast multitude of interconnected factors which all line up to form endurance. We are one complicated machine indeed!