Think about what five minutes could possibly yield in conditioning. Obviously you have to issue a challenge to your heart and lungs walking isn't going to help you.
Suppose you did the opposite of walking. Suppose you did wind sprints for five minutes, that is, you ran 40 meters in one direction as fast as you can- a full out sprint every step of the way, then immediately turned around and did it again in the opposite direction.
What you would find is you couldn't do it for five minutes. In fact, no-one can because as hard as you can exercise is shorthand for anaerobic and no one's body can process the waste products of anaerobic exercise fast enough to keep the body working anaerobically for five minutes. The result is, you stop and your heart and lungs are working as hard as they can.
So now you have a formula for working out hard in five minutes. No doubt your conditioning will improve over what it is now, the thing is, it's not clear it would improve as much as if you were able to dedicate 20 minutes or hard exercise interspersed with lower intensity exercise. 20 minutes 3 times a week is the minimum of time usually cited as needed for progressive improvement of your heart and lung capacity and efficiency.
Under that assumption, repeated runs at achieving the impossible- a full 5 minutes of continuous wind sprints- would not be as effective for conditioning. That said, I am not aware of any researchers who have investigated the effects of a such specific and limited regime.
You can get in a completely challenging (to say the least) workout in five minutes. Whether it's a workout that will lead to progressively increased fitness and if so, what that fitness might yield in terms of capacity and endurance outside of doing wind sprints, I am not sure. I am not advising you to do this (consult your doctor), I am merely pointing out that 5 minutes is actually plenty of time to give your heart and lungs a real workout.
Something to think about.