One thing that makes the NEC so successful is that they have provisions for both transit connections in the larger cities and park-and-ride facilities to appeal to suburbanites. Thus, if you live in the city of New York, Philadelphia, Washington, etc., there is lots of relatively convenient transit access to the stations. If you live in the 'burbs, you can board at Metro Park, BWI Airport station, New Carrolton, etc. Some smaller cities may have transit connections, but for a large part of the ridership, it's more convenient for them to just drive or take a taxi/ride share to the station. Baltimore is an example of this.
As far as comparing driving end to end and driving to the station to take the train, I could either get in my car, leave right from home and be in Center City Philadelphia in 2 hours or midtown Manhattan in 4 hours. Or I could drive 15 minutes to the station, probably wait around an average of 20 -30 minutes for my train, and take a 1:10 Northeast Regional train ride to 30th St. or a 2:40 Northeast Regional train ride to New York Penn. In both cases, even with the "last mile problem" and wait time for the train, the train is time-competitive with driving. This shows that a 75-80 mph end-to-end average speed is probably sufficient for corridor rail service if the goal is getting cars off the highway.
Of course, not all trips are between downtown and downtown. That means that if your final destination is in suburbia, it might well be faster to just drive the whole distance.
But there's more than just speed and travel times involved in a decision on whether to drive or let someone else do the driving for you for most of the distance. I can attest that driving I-95 between Baltimore and Philadelphia or New York is not an enjoyable experience. Even if the traffic is flowing freely, the volume of traffic, number of big-rig trucks, and aggressive drivers makes the drive very stressful. But there are usually traffic jams at at least one point along the way. And getting across the Hudson into New York almost always involves jammed up traffic. I think the last time I was able to ride into Manhattan without a traffic jam, LBJ was president and I wasn't old enough to drive. Then, of course, once you arrive at your final destination, you have to park your car. I think street parking for more than an hour or two is another thing from the past. (We don't even have it any more in Baltimore.) Thus, you have to go into a garage with rates that make the $20/day rate at the Baltimore Penn Station garage look like free parking.
Sure, corridor stations need to be easily accessible to local transportation. Stations in cities where a relatively large percentage of the population rides transit might benefit from transit connections. However, in most of the smaller cities and towns in corridors outside of the NEC (and also in parts of the NEC, too), what they really need to good convenient secure parking at reasonable rates, and also good access for taxis, ride share, and drop-offs. Because, let's face it, most of the Amtrak riders will be taking their cars or a taxi/ride share to the station, or be getting dropped off or picked up.