One bishop ran the show in one large town. As the faith spread, a bishop had more and more presybters (elders, priests) helping them, and they delegated more and more duties to them.
The idea that one town would have more than one congregation (or parish), with one (or more) presbyters serving each one, developed later on.
In the early days, of course, there were only enough Christians for a single congregation in even the biggest towns, and few or none in the smaller towns and countryside. (Our word "pagan" comes from a Latin term equating roughly to "country bumpkin".)
Alright, that sounds reasonable. When the word "hierarchal" was used I was thinking in terms of something comparable to today (very centralized government for the whole Church), but it sounds like that didn't happen for a while.