Since this system started in my town the max people we have managed to get to a raid was 11 (on the first Tyranitar that spawned). So no way we gonna be enough to beat a legendary. I really hope they are thinking of a system for lower density areas, but don't count on it.
Niantic does not care at all about small places, they have showed in the past and I am pretty sure they will show it again with legendaries.
And I don't think driving to the city is an option because as my experience most of the time people start raids as soon as the egg opens, I have get to Tyranitar's and Snorlax's 20 mins later and there was no one there already.
By the time I get to the city, park and get to the raid spot then everyone would have done it already. And of course the need of using a third party app would be necessary, do you know how contradictory you are in your own game Niantic? You don't want me to use third party apps to find the gyms where the raids are at, but at the same time is the only option you leave for a ton of us. Nice one!