The three hour thing is relative. I can comfortably knock out a sub-3 hour marathon on relatively minimal run training assuming the bike training is endurance-focussed. In fact I did that a few years ago due to an injury that prevented me from running until a few weeks before the marathon. To some extent the lungs and heart don’t need to know what the legs are doing if you have a long enough history as a runner.
When you’re looking to PB however, that’s a whole different thing. That takes true dedication to the running as @kurt.braeckel said. I’m off the bike other than a few times a month at the moment to focus on getting a PB in my February ‘19 marathon. I use an adapted version of the Jack Daniels’ method.
I have every confidence in TR’s bike plans, but for an open marathon I’d probably opt for one of the well-known author’s plans, or a variant of one.
One of the hardest parts of marathon training for me is the knowledge that my bike power is almost certainly declining - in fact I’ve noticed my leg muscles becoming visibly leaner. I do commute between 10 - 25k five times a week, which may go some small way to maintaining bike fitness.