Practice, then play
I’ve reached the conclusion that it’s fine to work at almost anything as long as you understand the difference between “playing” the guitar and “practising” the guitar. The latter involves identifying a specific aspect of technique – such as scales, arpeggios and so on – that requires attention, and then concentrating on that for a while.
By contrast, “playing the guitar is what you do when you’ve finished “practising”. If the two are confused it wastes valuable time, and doing so will also cause you to embed your deficiencies into your playing by continuous repetition: “practice makes perfect”, but practicing mistakes makes them perfect, too!