Workout thoughts

Hello all. Some background. I am 39 years old and about a year and a half ago was diagnosed with a chronic neurological disease that brought my exercise program to a screeching halt. I was fitness walking and using swimming as cross training and working on a marathon training program. My brain can no longer take the stress of heavy exercise. Swimming seems to be okay as long as I don't push it too terribly hard. I think the water keeping my body temp down must help. At any rate... My neurologist has cleared me for swimming as long as my head is behaving. My meds make my breathing rapid and shallow when I exercise so she's only allowing backstroke so I don't drown myself, haha. For the past three weeks, I've been swimming 1/4 mile of a warm up of a modified back stroke, then alternating laps of backstroke and kickboard. Since it has been a year and a half since I've been able to do any real exercise, that tires me out. The last couple of laps are not very straight in my lane... lol! I am wondering how to increase the distance. I think jumping to 1/2 mile is foolish given my situation. I am thinking maybe one day a week adding 1 -2 laps to one day. Then the next week, adding 1 -2 laps to another day, and doing the same until all three days are equal again and then starting that process over. If a week seems too hard, repeat the week and don't increase the next week. That was the strategy I had in the marathon training, but I don't know if that thought process translates the same into swimming. Is the idea of alternating laps of backstroke and kickboard sound? I want a leg workout as well as an upper body workout. Weight training is not allowed for me right now. Is it a better balance to do two laps of backstroke to one of kickboard? Or something? Thank you for your thoughts and advice! Pamela