Nob Akimoto wrote:
It was in Relics where Picard complained about the Stargazer being overworked and underpowered. But he also described her "always on the verge of flying apart". He was probably exaggerating a bit, and there's the whole fact that he was in command for well over 20 years. By the end of that commission, she probably hadn't seen a major refurbishment or rebuilding, which might explain Picard's belief in her being underpowered and overworked. Perhaps other Constellation class ships got new warp cores or other upgrades if they weren't on super deep space assignments like Stargazer.
That was the basis of my assumption that Starfleet explores in "tiers" that back fill as new frontiers open up. In the Constellation's heyday, the "five year mission" zone was the ragged edge of explored space; by the time Picard commands the Constellation, that same region is mostly farms, towns and industrial ports (notice the Enteprise-D never revisits any of the worlds the original Enterprise did; bet you two bars of gold pressed latinum that Picard had to run at least one peacekeeping mission to Sigma Iota-II).
"Overworked" because they're the workhorses of the fleet now; Starfleet is sending them to do literally everything from space patrol to law enforcement, anti-smuggling, safety inspections, search and rescue and so on. "Under powered" because the ship's original 2280s warp drive is barely up to speed with some of its civilian counterparts and Stargazer is sometimes in the awkward position of being outrun by suped-up smugglers.