You must be rushing the missions like mad dude, or do you do literally nothing in between missions just bang mission then straight into another mission, I always travel home to sell me loot, check if there's any better stuff for sale, see what potions the alchemist has now etc.
Heck I would say I've spent 51 hours just exploring all the dialogue options of npc to see what new missions and info they have for me, there's no way you've talked to people or read any books if you've done all that in 51 hours. How do you play missions?, when I'm exploring a cave and I see two ways to go I explore both I don't just explore the route that leads to the objective.
I fast travel everywhere and at 250 hours, if I explored everywhere instead of fast travelling it would be twice that figure. I've not finished any of those quest lines, I'm fascinated by the lore and I enjoy learning about the world I'm paying from its inhabitants, I guess your only interested in the action if your only doing missions.
The thing is, I do all those things. Every time my inventory is full (which is after each dungeon) I go to Whiterun to sell excess loot, check the merchants, train my Smithing and Enchanting and sometimes go on a thief raid across the town, as well as decorate my house with anything cool I can find and storeing the rest. Then when I have run out of interesting quest I pick a random direction and just go till I'm bored of raiding caves and giant camps... But the savefiles will not go over 50hours per character, max... In the last characters, that I stopped doing most questlines, I reach my limit at like 30-ish or below...