Hoping to encourage you with a bit of my own journey. Started riding last July. I was diagnosed as to being 70% effecient in my breathing. Still considered normal but 69% would of dropped me out of the normal range.
I rode a 24 speed Giant Roam at the start of my riding career. I was doing what is called mashing, pedalling slowly in a bit of high gear. Slight hills were killing me. I kept at it, learned to pedal and use the gears, found a loop to ride from my house that was 10km start to finish. I rode that everyday gradually pushing myself to go faster. Eventually I was doing it twice in a row. My breathing has greatly improved, though getting winded on long steep climbs still happens when I charge them. My regular loops now are 30 to 40 km long, and I average 19 to 20 km/hr. I'm 69.
By the way, I bought my bike because my doctor had really improved his weight and posture, and he said it was from riding a bike. Told me to get one so I did. Best advice I have had in a long long time.
If you are really concerned, check in with your doctor. If you are just unfit but healthy otherwise, start pushing yourself. It's not easy but it will pay off eventually.