'This river is as wide as five kilometres as it nears the sea'? This sentence is almost the same except the maximum width of five kilometers seems to be more definite.
'I walked as far as three miles' It is a bit awkward but I don't have a problem with this sentence. The person is not sure the exact distance that was walked but believes it to be three miles at the most. Personally I would probably use "about" instead of "as far as"
I walked about three miles.