We did the Muxia to Madrid trip in one day last week. Bus from Muxia at 6:15, then at train from Santiago to Madrid that arrived early afternoon. Used our Cercanias Combinado that (comes with the train ticket) gives you access to the Cercanias line in Madrid to get to our AirBNB apartment on the Gran Villa. If a university group from the US can do it, I think almost anyone can.
Do buy your ticket in advance if you can.
The train to Madrid was full and our bus was standing room only by the time we got to Santiago. We were the first and only ones boarding in Muxia. But that particular bus time picks up people working and going to school who live in the smaller communities outside Santiago.