I rarely run except on Main Street or during mini games on the island, but when I do I use the B button. I might try using the L button now to get thru the dialog with the animals and still be able to smash the A button to move the conversation along.
I don't think I posted on here yet, but I ALWAYS use the r button, I just find it so much more convenient when I use L to talk sometime and I accidentally don't want to mess stuff up with B, haha. =)
I always use L, I don't use A or B buttons when I have the option not to lol I hate the way those buttons start to feel "squished in" from overuse. Particularly the A button. :T