The New York Times committed a typo when it wrote (yesterday) about how Sooty Shearwater birds fly a 40,000 miles around the Pacific each year. Obviously there's something wrong with "a 40,000 miles" in the last sentence. We could fix the NYT's typo like this: ... Sooty Shearwater birds fly a good 40,000 miles around the Pacific each year.
"A good 40,000 miles" is decent, idiomatic English. But whatever the word "a" is doing in that idiom, it SHOULD perform the same function even when the word "good" isn't there. Shouldn't it?