How long is a long walk??

We have all heard that going for a long walk can get labour started, but how long are we talking here? I have been walking 3-4 miles regularly throughto pregnancy (2-3x per week) and now at 38w I am still comfortable to do 4miles in 60min. I'm thinking I'd have to do a half marathon or something stupid to actually make labour start! Does anyone know? If you've had a baby before, did you walk to start labour and how far? Thanks X