i ride 78" between 5 and 10 miles most days, and up to 30 (so far) on weekends.
i live in san francisco, i'm 33, i weigh 135, usually ride brakeless, and have had reconstructive surgery on both knees.

i also run about 20 miles a week.

hth

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.

I ride about 70", thats 15x39. Initially did it because I was going for a cheapo road conversion. The 39T chainring on the double crankset lined up exactly with the cog on the suzue basic hub, and I got a 15T cog because it was the smallest one the shop had. Works pretty well for my commute, back streets and a lot of stops, but I have been wanting to raise my inches for a while now...but I would probably still stay under 80".

i ride my three fixies around town as 48:16. each with 170mm cranks. one has 27" wheels, the other 700c. all brakeless. i would estimate my commute about 10 miles round trip, but i have never measured it. i love this ratio. i love that it's exactly 3:1. it's perfect for me and my terrain.

i built up a carlton raleigh as a ss for my brother recently. i set it up as 45:17 with 170 cranks and 27" wheels. i really loved the feel of that ratio. quick and fast.

inspired by the raleigh, i'm currently stripping my ross down to a single speed and am probably going to set it up as 40:17. i plan on mostly using it as a bad weather/trail bike, as i put fenders and knobby 27" tires on it before the tear down...

I know that feeling. I built up a bike for a friend at 40/16. At first I couldn't figure why his 25 year old S12-S was a lighter bike than my 2001 Fuji Track then I realized it was because I was used to running a 48/16 (now 48/17 because he has the wheel with the 16t cog).

Anyhow, I run a 40/17 on my mountain bike. Depending on what "bad weather" means to you , you might find it a bit slow. Mine's fine when ploughing through snow-covered streets but slow otherwise.

when i mean bad weather, i mean snow. if i hate it, i might change to a 44:17ish. i have the chainring and freewheel now for 40:17. plus, that gear inch will be close to 42:18... this guy brian, who runs the best bike shop here (all old weird used stuff for the most part) used to road race in the 70's on the olympic team. he said for training they would all go out on long rides involving super climbs on fixies running 42:18. sounded intriguing.