I'm not certain how bad your infection is, and I don't want to tell you to use O2 or not, but whatever you do start out SLOWLY. I had pneumonia two years ago, and I self-diagnosed as well as administered medication for it (TOBI), which fortunately solved the issue, but I was completely knocked down as far as running went for about two months. Even a normally 100% healthy person cannot jump right back into game mode with respiratory infections, and especially with CF you need to take it easy, and keep the progression slow but steady. Only you can really determine how your body feels, and if you can handle some exercise without supplemental O2 then try it, but again, keep it steady but slow. Simply focus the first few weeks upon achieving a cardio/weight training goal, then stay at the same level for about a month. If you are still able to handle it after a month, then add some weight, or add some speed/distance/time to your cardio (However, ONLY increase one aspect at a time, don't ever attempt to increase speed, and distance, and time all at once). Most importantly of all, please pay close attention to how you feel, and only keep at it so long as it makes you feel better.