[Avshalom]

>I suppose the real question is then "How do we decide what is 'worth doing' ourselves?"

Well that's the thing, Chesterton gives advice on that in the quote:

Is it worth doing even if you're doing it poorly? Then it's 'worth doing'. Is it only worth doing if you're doing it at a high/professional/competitive level? Then it's not 'worth doing'.

Of course as pointed out in every third comment, take Chesterton with a grain of salt. Chesterton never said anything that he didn't later contradict, and assert that contradicting himself was in fact proof of the ultimate correctness of both statements.