Dr. Atkins really doesn't recommend strenuous excercising such as what you've been doing during induction for the reasons that you experienced. It takes a couple of weeks for your body to get used to burning fat for energy instead of carbs.
I don't know if I'd recommend jumping right from induction to 50 grams of carb or more per day, especially if your goal is continued weight loss. You might try upping to 30 grams for the next week and see what happens.
If potassium is what you've been missing, you can either use a supplement or eat more veggies (yes, veggies) instead of fruit. Spinach, avocados and broccoli all contain much more potassium than a banana and are much lower in carbs and higher in other nutrients as well.
If you do decide to add fruit back in, start with berries such as strawberries, raspberries and blueberries as well as melons such as honeydew or canteloupe. All of those are lower in carbs and lower on the glycemic index than apples, oranges or bananas and higher in antioxidants too.