Hiruzen Sarutobi was named as the third hokage at a very young age! But later on, he became the jonin incharge of Tsunade , Jiraiya and orchimaru. Minato, the fourth hokage was the student of Jiraiya , so it wouldn't be possible for him to be the hokage during the sanin's initial phases as a ninja! So Hiruzen was both Hokage and a joinin incharge at the same time! How is that possible?
Either ways, he was hokage and their sensei rite? How could that be?
The hokage was initially a much less demanding role. It's likely that this changed with Tobirama, since he's well known for creating a number of things. Hiruzen delegated a lot to Danzo so he wasn't quite as busy. Tsunade was Hokage after the Konoha crush, which roughly halved Konoha's deployable forces while keeping the amount of missions the same.