Certified course vs Strava workout length

It's as close as any GPS-based system is - i.e. five runners will all have five different measurements (usually varying by a few hundredths of a mile).

Last year we had to run the first three races (10, 15 & 20km) of our Boston Buildup Winter Series as virtual events. We gave runners the option of running the actual course during the week prior to the normally scheduled date (we marked it for what would have been race weekend) or any route of their own choosing of the same distance. The Westport Summer Series did the same thing in 2020.

We kept two sets of results since the courses are quite hilly and it wouldn't have been fair for someone to run the same distance in Florida). Runners could use a GPS app called RaceJoy to record their time or upload from Strava or simply from a stopwatch (if they were running the actual course). It worked out fairly well we got about 50% of the usual turnout for a "real" race. But runners were glad to get back to the real thing this year.

Glad that worked for you.  It seems like there should be a way for more "virtual" events, but there will be more programming involved.  When I was thinking about the possibility, I was considering Colorado participants vs Florida participants, and simply the difference in altitude.  You now have brought up simple elevation gain/loss, which needs to be factored in.

Seems like a spreadsheet with programming/formulas is in order.

Good luck with it.  I think virtual events may be a growing segment of the market, especially if there is a factor that can be used for every 500' of elevation gain, or a way to factor-in vertical gain/loss, for instance.