First off--I'm sorry to hear of your illness. Your desire to work through the issue and retain your position with the PTO is admirable but I would suggest you consider turning your duties over completely so that whomever takes the reigns feels in full control to support the school the best way possible. We all have our own styles and approaches and someone temporarily filling in will probably do a good job but won't "own it" like they would if they knew they were in it for the long haul.
I'm very happy to hear your prognosis is positive with your illness, and recovery is of course your priority. Not having anything nagging at you on the back burner will give you the freedom to stay focused on getting 100 percent better and perhaps taking your position full time come next September.
I have the sense that approaching things from the position that you healthy is the best thing you can do for your family and your PTO. You can always become heavily involved when you're back on your feet...and then retake the reigns completely when you're at 100 percent.
I'm one of two volunteer coordinators for my school's PTO and I've recently been diagnosed with a semi-debilitating but treatable chronic health condition. It's been a struggle for my family as we try different medications to treat the health condition and adjust to our new life. Because of this, I asked for three months off from my position on the PTO to sort of regroup and get things back on track.
The trouble is, our PTO is young and this is the first time we've encountered a problem such as this - an officer experiences a short-term problem (illness, accident, death in the family, etc) and wants to return to his or her duties. We'd discussed a few options: filling my spot with an interim officer, asking my colleagues to simply pick up the slack, or requiring me to step down. We'd appreciate the perspective of PTO officers who have been in this situation before.