And in TOS "The Menagerie Part 1":
MENDEZ: You ever met Chris Pike?
KIRK: When he was promoted to Fleet Captain.
MENDEZ: About your age. Big, handsome man, vital, active.
KIRK: I took over the Enterprise from him. Spock served with him for several years.
SPOCK: Eleven years, four months, five days.
This shows that Pike was one of only two starship captains, both in the TOS era, known to have been promoted to the little known rank of fleet captain.
The other is seen in "Whom Gods Destroy":
SPOCK: A total of fifteen incurably insane out of billions is not what I would call an excessive figure. Who is the new inmate?
CORY: Garth. Garth of Izar, a former Starship fleet Captain.
(He calls up a picture on a monitor)
KIRK: When I was a cadet at the Academy, his exploits were required reading. He was one of my heroes. I'd like to see him.
KIRK: I agree there was a time when war was necessary, and you were our greatest warrior. I studied your victory at Axanar when I was a cadet. In fact it's still required reading at the Academy.
GARTH: As well it should be.
KIRK: Very well. But my first visit to Axanar was as a new fledged cadet on a peace mission.
GARTH: Peace mission! Politicians and weaklings!
KIRK: They were humanitarians and statesmen, and they had a dream. A dream that became a reality and spread throughout the stars, a dream that made Mister Spock and me brothers.
KIRK: No, sir. Captain Garth, Starship fleet Captain. That's an honourable title.
GARTH: And I was the greatest of them all, wasn't I?
KIRK: No, I, I can remember. You were the finest student at the Academy, the finest Starship Captain. You were the prototype, the model for the rest of us.
GARTH: Yes, I do remember that. It was a great responsibility, but one I was proud to bear.
The qualifications for becoming a fleet captain are unknown. But they seem to have been rarer than starship captains below them and commodores above them. If, repeat if, a captain had to be another Garth of Izar in order to be made fleet captain those were pretty big shoes to fill.
And one significant voyage of the Enterprise under Pike is mentioned in "the Menagerie Part 1".
KIRK: What every ship Captain knows. General Order 7, no vessel under any condition, emergency or otherwise, is to visit Talos Four.
MENDEZ: And to do so is the only death penalty left on our books. Only Fleet Command knows why. Not even this file explains that. (unlocks the magnetic strip) But it does name the only Earth ship that ever visited the planet.
KIRK: The Enterprise, commanded by Captain Christopher Pike.
MENDEZ: With a half Vulcan science officer named Spock.
And in "The Menagerie Part 2":
UHURA [OC]: Message from Starbase Eleven, sir. Received images from Talos Four. In view of historic importance of Captain Pike in space exploration, General Order Seven prohibiting contact Talos Four is suspended this occasion. No action contemplated against Spock. Proceed as you think best. Signed, Mendez, J.I., Commodore, Starbase Eleven.
This implies that Captain Pike's historic importance in space exploration is so very important that General Order Seven is temporarily suspended this one time. So Pike must have explored a lot of planets for the first time, make first contact with a lot of space travelling species and federations, negotiated the admission of a number of worlds to the Federation, defeated a lot of planet destroying space menaces, etc., etc., etc., and generally have been the James T. Kirk of the previous decade, just as Garth would have been the James T. Kirk of the previous generation.