Add MTB Route, how to select part of a road / path?

Hi all,

I have been adding some paths around our village and now I would like to add the local MTB routes. I do understand (I think) how to create one using https://wiki.openstreetmap.org/wiki/Cycle_routes#Relations There's one thing so far I'm unsure about. Let's say there's a main road (one "line") with a road connected to it half way, to the right. If the MTB route runs along the "main" road and then (half way) takes the connected road on the right, I don't know ow to select only the first part of the road. Am I supposed to split the road (the line) in two to be able to select only the first part of the road or am I missing something?

FYI I'm using the online OSM editor