When I quit smoking and started vaping, I started my daily workout back up because I felt better and noticed my lungs were clearing up and returning back to normal. I do 120 reps with free weights and a couple of sets on a Total Gym daily, then I ride my Weslo 10.8X stationary bike for 5 miles/ 25 minutes. That's about 12 MPH. I start at 60% resistance and work my way up to 90% and cool down at 50% for the last 5 minutes. That's equivalent to increasing the hill height incrementally over time and then backing down to a more level ride for the cool down period.
In the higher ranges where I'm pumping hard, I have to breathe a little harder to maintain 98% oxygen saturation, but that's normal. I'll find my self pumping and sweating, but not breathing hard. I keep a finger clip Oximeter next to the bike and put it on every few minutes to check my O-Sat reading. It's easy to stay in the 95% to 99% range under exercise stress these days. I just have to remember to breathe a little harder to stay in the higher range of O-Sat. It's funny that my brain is happy with 96% to 97% under heavy exercise and doesn't make me breathe harder.