COVID-19 infection potential
There's another study (European) which delves into the effects of strenuous exercise and the potential to become infected afterwards. Here's the reference:
One gist of this long document is that extreme exercise, while making the lungs more efficient for that exercise (e.g. marathon running, long-distance cycling), is likely a detriment to the body being able to deflect the virus naturally -- the virus is able to penetrate much more of the lungs. They state the vulnerable period AFTER the exercise is 3-72 hrs (while your body recovers).
As for the Cyclepaths, we generally do ride longer as the year progresses (we're up to ~52mi on the long routes already) which do take more than a couple of hours. To me the length of the ride is not so much the issue as is the amount of exertion you're putting on your body; i.e. if you're able to ride long and basically breathe from your nose most of the time, then you're not over-stressing your body. I'm certain there is some wiggle room in how much your lungs are expanding even though you don't feel over-stressed, but we're not marathoners nor Tour de France atheletes.
As is the usual case, the position you take on this issue is up to you. If you can tell you're over-stressing your body on a ride, according to this paper, you're likely to be increasing your risk of COVID-19 infection within 3 days after the event. If you're uncomfortable with this assessment, then you'll need to back off the exercise level and/or isolate yourself for ~3 days after the event. That's up to you.
Also keep in mind that pace-line cycling puts you in the "path of exhaust" of cyclists ahead of you; thus, you could be breathing in a sputum, unknowingly, from pace-line cyclists and that "event" could be the trigger within the following 3 days ... you/we just don't know.
As for the group that meets every Saturday, the following guideline still applies:
We ride upon our own personal cognizance, taking all risk upon our individual selves -- this COVID-19 situation is no different than any other Cyclepath ride in this regard; we're all aware of the increased risk in this situation.