If you haven't already, start jogging/running. Unless you are in a super-dense city it's easy to meet isolation requirements. Start jogging at the start of each 5 (or 2) minute boundary, jog until you're too puffed, then walk until next boundary. Repeat until 20, 30 or 45 minutes is up. Done. Then repeat every day or other day until you are jogging non-stop. Congratulations, you're now a runner and your lungs are now unavoidably stronger.
Don't bother with stretching rituals unless you have extra time to burn -- they don't really make a difference versus simply running within your current comfort zone. Half of the running biz is a meditation on the state of your body during and between runs (the other half is scheduling, and that's aaaaall different now.)
Also, if you have 'gunky lungs' (either standard cold/flu or COVID-19) wait until you are mostly gunk-free before running!
I'm a daily runner now for 26 years ( five miles a day currently) and an asthmatic who always has gunky lungs, but, as per my lung test last year, I have the lung health of someone 20 years younger instead of 20 years older, which is what my doctor expected given the severity of my allergies and asthma. I grew up in the 80s when I was forbidden from all activity because of my asthma, but running is my bulwark of breath, my mental/physical/meditation on how to breath healthy instead of in a diseased asthmatic way.
I remember a paper that found that its worse to run near roads than to run at all, with huge amounts of absorption of contaminants into the body. Contaminants being "recorded" in the runner's hair.
Out of the blue I found this paper finding a relationship between worse performance and pollution in marathons.[1]
That study was done in China which is pretty notorious for its air quality. Probably best not to run outside when the AQI is high though. https://airnow.gov/
Anecdotally I noticed a difference when running next to big, busy roads in Northern VA vs. running next to lakes (or even the same busy roads later at night when traffic was negligible). Couldn't go as hard, didn't feel like I was making as many improvements to speed/time/health, etc.
Don't bother with stretching rituals unless you have extra time to burn -- they don't really make a difference versus simply running within your current comfort zone. Half of the running biz is a meditation on the state of your body during and between runs (the other half is scheduling, and that's aaaaall different now.)
Also, if you have 'gunky lungs' (either standard cold/flu or COVID-19) wait until you are mostly gunk-free before running!