I am in decent shape and ride maybe 300-500 miles per month. Ride to work and one long fun ride each weekend. Last fall I hurt myself in such a way that I could not ride to work. I couldn't use the bus because I could not stand and not disabled enough to get a seat. I could not drive my car. Taxi, lyft, uber was too expensive. My only option was walk to work.

I walked at a pace of 13:30-14:00 minutes per mile and my HR was about 20 BPM lower than what I would expect when riding my bike at a moderate fast pace. Moderate fast pace would be something I could sustain for a very long time, like hours without going out of breath. I could talk ok and maybe hold conversation, but maybe I could not sing.

There is 300 ft hill climb for the last 1/2 mile and I kept the same pace. My HR increased by about 10 BPM.

Actually maybe I was walking faster because I had to stop all the time for crossing the street and such.

But damn my legs were tired from the 7 miles of walking each day.

What to get out of this...I think your respiration and circulation/heart systems will be in good shape to support you. But without much riding, your muscles will be holding you back until you develop those.