I have been running 42*18 and my brother rides 39*16. Both of us are more interested in fitness and rpms than top speed.
My brother just returned from an interesting trip. Fly to Rome, bus to Naples, cycle down the Amalfi coast for 8 days to Sorrento, bus back. He was riding 8 plus hours a day up and down mountains. He said a taller gear would have been nice going downhill, but 'HE' could not have pushed it uphill. PS. He is 55.
Gearing is dependant on needs and geography. An answer from Chicago is is next to useless for someone from Bend. Also your need is rehab, not to prove something to anyone. I advise a lower gear and work on spinning to develop strength both in the muscles and heart.