It’s my observation that ambulances covering events are almost always parked at the wrong location relative to where people are likely to become sick or injured. That’s true if the event is a walk, run, ride, etc., that occurs over a distance. This isn’t such an issue for events at a fixed location.
Each event is different, but on an out-and-back route, placing the rig at the start/finish line means an incident at the turn-around could be very far away. Especially, if you have other medical coverage at start/finish, the rig really belongs out on the course. Perhaps at a spot where accidents are most likely.
This is not one-size-fits-all. In our case, we moved the rig from start/finish about a third the way out on the course in a spot where the ambulance could quickly get started in any direction. (It was parked on a cross street where we had volunteers stationed and could drive on the closed streets used as the course with little danger).
Hams don’t often get involved in these decisions, but event organizers may not think about these things and the ambulance crew may not understand the specific hazards of the event where they are standing-by. So, it’s worth thinking about, especially if you will be assigning a radio or a ham to the ambulance.