Getting into the state of 1 thought was very easy for me 20 years ago:
Run 6 x 300m at 80% effort with a 90 seconds recovery between reps.
"I want to die" is the only thought remaining at the climax of the session
And if the session is modified to 6 x 150m up a grass hill on a cold, foggy November evening, the state of no thought is reached by about the 5th rep ;-)