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I was thinking it’s entirely possible for him to just live stream the whole run.

Heck, he could run a marathon on his own this weekend and live stream it.

All the serious runners I know have their own GPS watches already, and chest straps to monitor heart rate. I only know a couple though, so it could be a biased sample. Or perhaps it is an age group difference, but I would think a doctor who is serious about his training would be a prime candidate for buying one though.

Most serious amateur athletes now use GPS trackers but many elite pro marathoners still compete with nothing or just a simple digital wristwatch.

They’ll want an observer because “GPS watch malfunction” will be the next excuse. It does happen.

Great, then you don't get an official time. For noncheaters they can request a pacer as well.

He could also do the Yasso 800s for ex, no need to even run a full marathon to estimate pretty well somebody's time.

Yasso 800s are a good predictor but don't substitute for an actual time trial.

Observer is way cheaper.