It was almost nothing aside from the shortness of breath, although the loss of smell was distressing. Very mild soreness, minor weird and unproductive cough. The shortness of breath was unlike any sickness I've ever had and got me to go to the emergency room when I was scared of passing out. By the time I got there (30+ min; had to get picked up) my o2 was in the low 90s which is not much cause for concern.

The trick of it is that if breathing difficulty is the only symptom, it's very difficult to diagnose. You breathe faster and your heart beats faster to keep your o2 at 98+. In the emergency room there's no way to tell me (50% breathing capacity, 94 o2 sat, 120 bpm) from someone scared about losing their sense of smell (98 o2, 120 bpm, 100% breathing). The only way to actually distinguish is the amount of scarring on a CT scan (which is very rough, varies person to person, and really needs a prior CT) or a diffusion test, which requires the person to be well enough to breathe into a big machine for 15 or so minutes.

Since so few people have gotten such a workup, AFAIK there's no real sense of how severe the normal infection is. I would guess myself to be in the 80-95th percentile for unluckiness at my age. If the impact on my breathing had been just a bit better, I think I would have just acclimated to the new normal and called it a lingering cough. The "sick"-like effects were so mild that I would have otherwise guessed it was unrelated. Then again I'm prone to ignore stuff like that.

I deeply hope doctors keep a close eye on how the average person's lungs are affected in terms of gas exchange and scarring. Even if you don't notice a change, good breathing habits can help reduce arterial pressure and reduce strain on your heart.