Here is the back-drop. He talked big before WS 2016, but then went off-course. He talked big again before WS 2017, blew-up and DNF'd. He finally got it right in 2018 and 2019. So he has learned and is more mature now. However, really, especially without a coach, what are his realistic chances in the road-marathon which is a different type of 'pain-cave' that the article said he wants to go to?
Think of a simple analogy, like a 'hold-your-breath the longest' contest. A noob guy with a big aerobic engine is still going to lose to the guys that have experience practicing holding their breath. Similarly, if Walmsley wants to really get the best out of himself and hit the marathon's 'pain cave', then he should have been practicing running sub-5 min for long duration, to see how long he can sustain it while staying relaxed.
It is pretty late in the game to get much practice at that and keep fresh legs, but he can still get some practice in, including meditation on it. Part of success will be in being able to relax, focus, and eliminate the energy-absorbing distractions, to get into that rhythmic zone. Since he is a noob, he should be keying off comparable runners that have the pacing figured out.