HTupolev

If you're riding at low intensity either way, then adding breaks won't make you stronger. But if the question is thirty miles of aggressive 8-mile "intervals" with regroups, versus thirty miles of steady, things are more complicated.

I've done century rides with no "stamina" issues at times when the other rides I'd been doing were mostly forty miles or less with frequent regroups.

There are good reasons to do long rides while training for a century, such as sorting out fit and nutrition. But from a pure pedaling stamina point of view, you really don't need to ride long rides non-stop in order to ride longer rides non-stop.