Well just to reassure a few of you....
I started riding 4 months ago at 52. The only exercise I ever did before that was walking about 3 miles a day for about 8 months before starting biking. Despite all that walking...
Nothing prepared me for how out of breath I got when biking up hills in the beginning. I was literally gasping for air, LOUD gasping breaths, and my heart pounding. This happened often, on most hills except for the smallest ones.
My DH (who usually rode with me back in the beginning) was SO patient while I kept stopping over and over to catch my breath and let my heart slow down. Like some of you, i suspected maybe I had asthma too, or heart disease or something!
As the weeks went by though, I gradually began to get less out of breath. My heart didn't race as much. After 2 months I could pedal up some hills I had to walk up before. I needed to stop and rest less often. My awful gasping slowly became just heavy labored breathing.
Now after 4 months, there are just a few hills around where I live that I can't pedal up, *mostly* without stopping. My heartbeat never feels uncomfortably pounding. My labored breathing has now become more like just deep mouth breathing.
And when going up steep hills, it's now usually my legs that will limit me rather than my breathlessness. That's a good thing I figure! I know I'm still not even halfway yet to the fitness level I would like to be at- maybe next year!
My DH and I would like to do some touring next year to visit friends in MA, VT, and NH. Right now my longest rides are 40 miles. When I can do 50 or 60 for several days in a row, I think I will be ready for that. Only a few months ago I was exhausted after 10 miles.
Just keep pedalling at whatever level you can. Pedal, pedal, pedal- it happens slowly...really!