I'm sorry to say that there was no quick way to do this one. Even circling the whole map did nothing to majorly contribute to the 100km. The best I can say is to find a safe space where you can run in a big circle and set up your controller with rubber bands and tape.
I used "Make Out Point" located above the lighthouse, but you can also go to the skate park that you unlock once you're in good standings with the greasers. Beat up everyone and locate a space that you can bike in circles until Jimmy passes out, and then return the next day to continue. You'll have to wait for the bodies to fade out, but if you look away they'll disappear faster and not return. Adjust their bikes as needed.
Ensure that you check in on this one; there is no way to walk away from this game because of the "passed out" function, make sure you have time for this.