I agree to all sugestions here, but I have a bit of another experience than kneeds considering running. For me running became a great way of stabilizing my stamina. With the right shoes and technique it is much less stressing for your knees than ballet often is, you just have to start carefully and if you feel uncertain, in any good sports shop where you can buy good shoes, they would also give you a lesson on the threadmill.
The advantage of running (or biking) is that the moment you leave the door of your house, you can start the training. When going to the swiming pool, you have to get there, change, shower, etc.
And the barre at the citchen counter is of course a must
Enjoy your summer course.