I found that this can be done during one of my running endeavors. I could just keep running without resorting to walking. It wasn't a 8mph sprint, it was a 4mph leisurely jog. It's better than walking though, at least for exercising efficiency. I go for runs every night at like 3 in the morning because that's usually when it's nice and cool outside. To acquire the ability to indefinitely jog, you need to go for jogs and runs every night.