Or they are so weak they can only walk 30 minutes per day and had to rest until the next day

5km/h is also faster than average walk that is probably not a pace you'd maintain for hours? After considering you also need to stop on intersection you may be getting the average down to less than 4, but then if it's actually a 10 minute ride you'd probably also spend enough time in stop lights and your average speed would also be way lower than 50km/h