We often use 1/4 mile (400 meters for my SI-using allies) as the walk-shed for transit. This is too short. See e.g. these graphics from the Snelling Arterial BRT study, which draws radii around stops. 1/4 mile does not even get you from one end of Rosedale Mall (which isn’t even the biggest Mall in the Twin Cities) to the other, and many people make a full circuit, on two floors, inside the mall, on foot. If we have nice enough environments, we should expect people to walk a 1/2 mile to 1 mile with no problem, shopping mall developers do, and they are far more mercenary than the public sector.
A longer assumed walk-shed has many advantages. It allows us to increase spacing between stops, which increases running speed, which makes transit more attractive for those already on-board. We always trade-off running time for access time (higher access time for lower running time, e.g. when we space stops farther apart.)
Jarrett Walker at Human Transit, discusses the issue and notes that in many urban areas there is no need to walk farther than 400 m, so we don’t know what people would do. He also notes the difference between radiuses and network distances.
Schlossberg and Agrawal also discuss this in: How Far, By Which Route, and Why? A Spatial Analysis of Pedestrian Preference. They find the average pedestrian trip to a rail station was 0.47 miles (nearly 800 m). Guerra, Cervero, and Tischler ask “The Half-Mile Circle: Does It Best Represent Transit Station Catchments?” and argue it is useful (and a slightly better predictor) for the residence end of trips, though shorter distances (1/4 mile) at the work-end makes are slightly better predictors.
In short, I believe people will walk longer than we typically credit if we can make decent walkable urban environments, environments which lead people to under-estimate the actual time involved (as the saying goes: time flies when you are having fun) because their mind is not on how awful the walk is, but about how interesting the environment is.