Not my personal challenge, I’m just on the support team for the day. They’re currently underway and can be followed at https://twitter.com/allthebuses
It is “one stop of each bus route”, the map is misleading, as we don’t have the precise information from the challengers about which stops they’re riding from/to, so the entire route is used as a (visually more impressive) approximation.
The selection of routes they’re riding are the all-year-around TfL routes that are running today, primarily excluding school buses (as they often only have 1 or 2 runs in the day, and all helpfully conflict). Additionally, a Thursday was chosen to avoid the 965 and 969 routes. Even so, there’s 593 routes, to complete them all individually in a day is impossible, they’ve split into 5 teams to cover them all, which should be just about possible. A previous attempt a few months ago fell short by about 20 routes.
Our timings sheet will likely be made public in full once it’s finished tomorrow, but the in progress timings and route allocations are visible in raw form through the Map’s API (I’m planning to make use of them at some point, but I’m out of time for this project today).