The Marathoner badge is for "Sprinting for an hour (3600 seconds)"
This is cumulative and all time spent sprinting contributes towards it. There are a few perks that improve stamina etc, that will make this a little easier but the best bet is to just load into a trial and sprint in circles.
Also two handy things to note:
1. The in-game badge menu tracks your progress.
2. Leaving a trial after gaining progress still counts towards it.