We took the ALSA bus back the other way last year from Gijon to Bilbao after finishing our two weeks on the Norte. It's kind of wistful to bus back in a couple of hours what you've taken days and weeks to walk.
If you're comfortable with the ALSA app (some parts in Spanish) and you've got a credit card or PayPal that you can plug in, it is very convenient and we usually purchase our bus tickets a couple days in advance just in case the bus fills up. Of the maybe 8 or 10 times we've taken the bus, maybe 2 or 3 of those were full, so probably good to buy a few days early.
If you're not comfortable with the app, you can book online too and print a paper ticket at home, just keep in mind payment is finicky and you may have to resort to PayPal.