But based on what we see in ENT (100 years after FC), Earth doesn't really have much of a "fleet," and they certainly haven't done much exploring.
Which is the point: ENT shouldn't have done that, because now it's the odd man out.
However, ENT doesn't exactly contradict the idea of "fleets of ships exploring", as it does make mention of all sorts of ship activity, including fleets of tramps exploring business opportunities. ENT merely establishes that all that effort amounted to nothing much - it took a warp five engine to actually reach points of any interest. Which I think is a fun thing to bring up.
We saw only a handful of pre-NX class ships, and two NX's. And those pre-NX class ships could only go Warp 2.
Or then warp 4. The only speed limitation we hear of is for the civilian freighters, and those
still do only warp 2 in the TOS era ("Friday's Child"). (Oh, and for the early versions of the Warp Five Engine, but that adventure never established or suggested that other ships of the era were doing as poorly as the test rigs.)
Whatever the nature of Earth's "fleet of ships", we know that there was a Starfleet in place where many officers reached high rank decades before the show. Via ship assignments or shore jobs, we don't know for sure. But there's no real mention of
lack of starships before NX-01, either.
What this choice of time frame means is that Earth has a full century to come up with tech that is at least roughly on par with the local interstellar standards, making it a bit more plausible that our heroes could survive beyond their first episode. But this also means that Archer has all the same goodies as Kirk, these apparently being the minimum requirement for survival in the interstellar environment. So we're left to wonder how there was progress between the 2060s and 2150s but none between the 2150s and the 2260s.
Would that issue be alleviated by a 2070s or 2110s show? If we believe in minimum requirements, this would then mean even longer stagnation, but that's plausible if humans get "on par" by buying or begging from Vulcans and make little indigenous progress. If we ditch the minimum requirements idea and allow Archer to venture into space armed with a Steyr machine gun borrowed from Lily Sloane, parachuting down to planets in capsules that have just enough fuel for one takeoff, we do get a show rather different from all other Star Trek... But while it complicates the technological aspects of writing and threatens to slow down the stories (the transporter was invented for a reason!), it's unlikely to affect any of the other, "continuity" issues.
Timo Saloniemi