[notacoward]

> People who push their bodies to the limit tend to get sick more often

Yes for extreme exercise, but "5K regularly" isn't going to trigger that effect. In that range, there's a positive long-term effect. That's what athletes and (even more importantly) physiologists really know.

https://pubmed.ncbi.nlm.nih.gov/32139352/

The problem here is far more likely to be the "in the gym" part. Being one of many heavy-breathing people in an enclosed space for an extended period will far outweigh any long-term immunity benefit from exercise. Also, OP mentions having COVID before. This raises the possibility of post-COVID inflammation and adaptive-immune-system hyperactivity due to continued RNA shedding. As if that weren't enough, there's mounting evidence that COVID causes lasting damage to the innate immune system. Like the "immune amnesia" associated with measles, COVID seems to leave people more susceptible to all kinds of infection, perhaps permanently.

My suggestion would be to keep running, but do it outside. Yes, even in winter. That's what I do myself, in New England, so it's clearly possible. You just have to gear up properly and learn a few extra habits to keep yourself safe.