fewer or less followed by "than"

Hi-Welcome to the forum!

I think in the first two sentences you mentioned 'less' is referring to some 'amounts'; therefore it IS correct. In your first sentence, for example, it is referring to 'distance'.

I run (a distance) less than 5 miles.

But about the second two, I was doubtful, so, I just checked Longman Dictionary of Contemporary English. It has a Usage Note at 'few' entry, which I copied the related part here:

!! Sometimes people use less before a plural noun, but many people think that this is incorrect, so it is better to use fewer • a village of fewer (NOT less) than 200 inhabitants

So you have FEWER than 5 apples. We could also get it from this: we never say, "You have less apples than 5", but "You have fewer apples than 5".

Hey, but I got a bit baffled!! What's the difference between this last sentence and the first two examples? I'm pretty sure they're correct though!! So..., maybe I was wrong! 🙂

Any time you are referring to a measurement it is an exception to the usual rule and is called "non-countable." I have read some explanations that supposedly show how these are in fact "non-countable" but I have trouble understanding much less explaining.   So I just remember that any time I am talking about distance, weight, time, money or any other common way of measuring something it is "non-countable" or I like to say a "mass noun."

With that in mind, your first two sentences, referring to measurement systems, are mass nouns and less should be used as you did.

With the last two sentences, there is no difference, grammatically speaking, between having apples and eating apples so since apples are something that can be counted and there is no exception to apply, "fewer" should be used as you did in the fourth sentence.

The third sentence is wrong.

Only the last sentence is correct.   Apples and miles can be counted, so we use "fewer" when referring to them.

Think of this: At 5 miles daily, I run fewer miles than your 10.   I run less than you do every day.  

I run less than you run.   I run fewer miles.

Eating 3 apples a day, I eat fewer than your 4 a day.   I generally eat less than you do at meal time.

Dick said
Any time you are referring to a measurement it is an exception to the usual rule and is called "non-countable." I have read some explanations that supposedly show how these are in fact "non-countable" but I have trouble understanding much less explaining.   So I just remember that any time I am talking about distance, weight, time, money or any other common way of measuring something it is "non-countable" or I like to say a "mass noun."

I guess I should not make a statement like this without giving references, so here are some sites I googled which talk about the exception to this rule for measurements. That would include miles and dollars in the first two sentences.

 < http://afterdeadline.blogs.nytimes.com/2011/03/01/fewer-vs-less/

 < http://grammar.quickanddirtytips.com/less-versus-fewer.aspx

 < http://data.grammarbook.com/blog/definitions/fewer-v-less/

 So I go with saying that "less" is correct in the first two sentences.