The stretches are a great idea and will probably help a lot. I would suggest strengthening your calves also, if you aren't already doing that. Sometimes when body parts fatigue easily, it means that something isn't as strong as it should be, and weaker muscles are trying to compensate. Maybe try a test and see how many single-leg slow calf raises you are able to do (do them on a step holding onto something for balance, so you can get full range of motion). If you can't do very many, it would be a good idea to do 2-3 sets 3 times a week to build up the strength, and see if that helps your performance in your cardio workouts.
Stebby