Real world transit-oriented design in the US routinely uses either 1/4 mile or 1/2 mile for the radius from transit that most people can/will walk. I believe they estimate that it takes 15 minutes to walk a quarter mile and 30 minutes to walk half a mile.
Single data point: I'm a pedestrian. I haven't owned a car in over a decade. I'm willing to walk up to 30 minutes regularly and farther occasionally. I also use public transit sometimes.
No, that's little old ladies in street clothes hobbling as fast as they can.
People walking as a mode of transit aren't competing with joggers for who gets there first. These times are for planning purposes.
Of course, it's okay if you walk faster than that. But planning departments need to look at "Who will actually walk this?" And the answer is "Ordinary people will walk it if it isn't over 30 minutes, but 15 minutes is better. And that works at these distances."
You see the most traffic from establishments within a quarter mile, and some additional traffic within the half mile radius but outside the quarter mile. It drops off steeply outside of the half mile radius.
Ah, makes sense. 30 minutes is a very slowly walked city mile for me (25min at a casual, unhurried pace, shaving a couple minutes off that for a 22-23min time with a bit more bounce in the step but still not jogging or even speed-walking) so I figured it was either round-trip or the slowest speed anyone capable of walking anywhere at all would attain—seems it's the latter.
Single data point: I'm a pedestrian. I haven't owned a car in over a decade. I'm willing to walk up to 30 minutes regularly and farther occasionally. I also use public transit sometimes.