A lot of the main quest's also have side, or "radiant" quest's tied to them. Red Eagle is one that comes to mind, as it takes some doing. The Artherial War's is a book you'd pick up during a quest that leads you to the Artherial Crown, and that takes some time to work.
Pretty much all of the quest's going into the Dwemer building's take time, and take even longer if you explore. Chase after the Word's, just make a lot of save's before talking to Arngier in case he sends you to Arcwind Point. There's a fairly long and hard one to do with the Shrine of Peryite. There's lot's after getting you char to be Master of a Guild, Arch-Mage, Vamp Lord, Listener.
I had my char just buy Breezehome in Whiterun, but all I did was put the bed in it, and went there to stash loot, or collect loot to sell. Shrine of Azura, Mehrune's Razor and on. There's lot's to do besides the Mage quest, and major decision's as well. How far will you have you char go? What line will you have it cross that separates good and evil to accomplish the quest's?