So Hashirama died when she was 4 then "Tobirama was appointed as the Second Hokage. During his reign he was primarily responsible for establishing Konoha's organisational system. He established various organisations such as the Academy, the ANBU, the Chūnin Exams and the Konoha Military Police Force. At some point during his era, he and the Second Raikage met to form a formal alliance between Kumogakure and Konohagakure; however, Kinkaku and Ginkaku, Kumo's Gold and Silver Brothers, attempted a coup d’etat against the Second Raikage and left Tobirama near death during the fiasco". Then the First Shinobi War started and he died before it even ended. Hiruzen became Hokage won the war managed to mature and grow a beard all that in less than a year. Yeah that seems likely.
Seriously though, I don't see the problem here.
First of all we can only guess about Tsunade's age in Hashi's flashback, she could be anywhere from two to four, so that gives us an additional two year span.
Second of all, even if we consider she's four years old back then, it gives us a two year time span, not less than a year.
Lastly, it has been nearly a year (give or take) since Naruto returned to the village and do you want me to list the happenings in that short time span?
The Kazekage dies and is saved by Konoha nin, they battle Akatsuki numerous times, Konoha is attacked, Konoha is destroyed down to the rubble, Pein is defeated, they build Konoha back from scratch, Tsunade is in a coma, Danzo is (temporarily) appointed as Hokage, Danzo dies, Tsunade reclaims her poisition, a Kage summit is held and the biggest war ever is waged.
Just two days into the war the alliance has fought zombie previous Kage's, bosses, Gin-Kin brothers, sealed the Gin-Kin brothers and is currently battling Madara, Obito and the bloody Juubi and I assure you that this fight won't drag out till nightfall.
On the other hand, the four previous Hokage's have been zombified by the likes of one sick b*stard named Orochimaru who was supposed to be dead.
The above paragraph has happened mostly in one day.
Just because Kishi likes to drag out instances doesn't mean they actually last that long. Every manga we read is about five minutes in Narutoverse, some more, some less. So it's perfectly plausible to consider all that you listed in your post happening in a time frame of two to four years. :|