I'm defining easy as 6:30 and slower, and hard as 5:45 and faster for elite and sub-elites. Granted Rupp's easy might be 5:20 while Ryan Vail's might be 5:45 for the same effort.
The question is not really about Rupp. It's about the complete lack of depth in US marathon runners. Other than Rupp, we suck. Back in the 70's/80's 2:12 was probably considered the minimal result to be considered elite. Malmo ran 2:12 something. He was not considered great (at the marathon). His 1/2 marathon time was great , but if you were asking opinions about his marathoning back then the consensus would have been that he was just not a marathoner. He was "ok", but better as shorter road races.
Today, he'd be one of our best. A possible Olympic team candidate. Yet today's runner would kill him on the track (well, he'd be decent in the steeple today on his best days).
Chris Derrick has run 13:08/27:30 (and probably could improve on that 10k time), but he's slower than Malmo in the marathon. He's slower than Kirk Pfeffer - a 28:20 10k guy. He's slower than Tony Sandoval a 27:47 guy. He's slower than Ed Mendoza. He's slower than Ron Tab, Benji Durden, Mark Conover.
The list goes on. There's probably 25-50 runners from that era with faster PR's than Derrick.
Heck, he barely beats Tim Varley - a guy who never broke 29:00 in a 10k.
I don't know how Chris Derrick trains. I don't know why he's not at least a 2:10 guy. He should be. But I'd wager than all those guys listed above ran their average easy runs faster than most elites run their easy runs today. That's may have nothing to do with it, but there's got to be some reason why no one (other than Rupp and Meb) can run a decent marathon today. The women are good, but the men suck.