On the other hand look at Kirk's behaviour in third season in "Requiem For Methuselah" or even "The Paradise Syndrome." What if "The Cage" story had been filmed then when Kirk could indeed be tired after years of deep space missions? Maybe then he could have been tempted by someone like Vina. After all in some respects Rayna or Miramanee were just as much an illusion of a different life to Kirk that Vina was to Pike.
I agree, especially since Shatner himself seems to be tired sometimes in Season 3.

The Cage is indeed a season 3 or 4 for Pike since he's so closed to burnout.
Like TREK_GOD_1 says, The Cage is really self-contained. For me, it looks like a one shot episode for an Anthology series or a short-story. But if we isolate The Paradise Syndrome or even The City on the Edge of Forever, we have a potentially self-contained story without any hints the Captain is ready to pursue his adventures in space.