I think surface=paved or surface=rubber could be appropriate.  However, unless the rubber is particularly soft and springy like playground material, the generic surface=paved might be a more  useful descriptor of the functional condition of the track.

https://wiki.openstreetmap.org/wiki/Key:surface

Either way, as someone with two very different bikes for on and off road use, I appreciate your observation of the surface of a "cycleway"!