No, it's not "1000s of variables" - just one - the ability to develop peak power that assures you remain in contact regardless of wind or incline.

"average speed would one need to keep in order to stay with the average pack"
Meaningless phrase, without knowing how hilly or windy.

What this means, you may need the ability to average 23mph by yourself if you want to stay with the "front" on a hilly course.

You may be able to finish "in the field" , even if you can only average 20mph by yourself -- as long as the race course is flat and not windy......