Thanks everyone for your great thoughts and suggestions.
The GPS part seems to be the easy part of the equation when I follow the group's ideas and my own research.
The trickier part seemed to be to having an easy way to have others follow along with the runners progress in real time.
Right now in the lead is Google Maps.
The runner can activate Location Sharing and obtain a link that could be shared via a social post or website.
The one downside I see is that to get a stable link, the runner will turn on Location Sharing and the sharing starts right away. So if you are promoting ahead of the event, people will be able follow the runner around the week before he actually runs. Or the runner activates the sharing just before the run and shares the link at the start of the run. Either way, the runner needs to remember to stop sharing after the event to stop people from following them for the rest of his life :)
Some of the other options for sharing (e.g. WhatsApp, Garmin, etc) had limitations, such as a small number of viewers, having people logging into specific apps, or having to directly invite people to view. Google Maps seemed the easiest so far just by having a link shared and clicked on.
Thanks again. Any other ideas out there, I would be happy to hear more.
Rod