We all know the difference between Less and Fewer now, right? It’s all about things versus stuff, as I explained here.
So what on earth is going on with the phrase, “One less thing to worry about”? And what about, “We’d gone less than 30 miles when we ran out of gas”?
Well, it seems to depend who you ask. “One less thing” is definitely right; NOBODY says “one fewer thing”, but it’s up to you whether you say that’s because it’s just weird English language idiom, or because even with THINGS, one isn’t really countable in the same ways as bigger numbers, so “one less” is OK for THINGS as well as stuff. (Anyway, one less is never going to be stuff, because by definition you can’t have one stuff).
As for distances, measures and so on, this is where my Maths lesson about discrete numbers and non-discrete numbers might have helped. I think of it this way:
“We went less than thirty miles” might mean 29 or 28 miles. Those things are countable, so I’d want FEWER than thirty of those THINGS we call miles. But realistically, it probably means 29.65234 miles, because miles aren’t discrete, so we’re really talking about distance, which is STUFF. “We went less far” would definitely be right.
The same with “He owes me less than thirty pounds.” Fewer would just sound weird, because it individuates the pounds, where the issue is really the STUFF that is money.
But explain it to yourself however you like, or just learn it as a rule, when you’re talking about measures, like money, weights and distances, less is sometimes more. Or fewer.