Complex sentence: Nabs initially started to cycle the course himself but gave up roughly half way, and it is presumed that he'helped' the effort of Colonel Ryan from the comfort of the support vehicle.
Split into simpler sentences: Nabs initially started to cycle the course himself but gave up roughly half way. It is not quite clear how he'helped' the effort of Colonel Ryan thereafter, but it is assumed that it was from the comfort of the support vehicle.