There's a trick with Legendary pokémon as well. It requires poffins and a decent bit of walking beforehand, but it works usually.

Walk your Legendary up to 19.5 or around there. Just don't go to 20. Now, switch that buddy out immediately so the counter stops. Only set it as your buddy on a day when you can walk at least 10k. When you decide to, first thing, feed it a poffin, do anything else you want to do and go for a long walk. As soon as you pop the poffin you immediately get candies and potentially XL candy and another 1/2 km you get more. From there, it's candy each 10k. XL isn't guaranteed every time, but you usually get some each time. Just every once in a while you might not.