Group size

Poetry can be challenging to translate because of nuance. “Ir” is “to go”, while “llegar” is more like, “to attain, achieve, or reach”, yes? So it is possible that the poet may not be speaking exactly about distance. You may be able to go faster by yourself, but if you want to achieve more, then that often benefits from help.

Something like that, maybe? I dunno… my Spanish-fu is pretty weak.

Fast solo, far with others I think is generally true. When someone struggles, someone else can help them keep going. An observation  from ~3000y ago:

Two are better than one, because they have a good reward for their toil. For if they fall, one will lift up his fellow. But woe to him who is alone when he falls and has not another to lift him up! Again, if two lie together, they keep warm, but how can one keep warm alone? And though a man might prevail against one who is alone, two will withstand him—a threefold cord is not quickly broken.