There are lots of things American that I don't understand and 'blocks' is one of them.
If a book says, 'he ran 8 blocks' it means nothing to me - roughly how far is 8 blocks?
barsel: "Since when has block been American? " - as with all things it was first British, however the answers are in the context of the OP asking about the American usage.
Yes, I have said, "I'm going round the block" when I was a kid on my bike but I have never heard anyone in the UK say 'the office is three blocks away' or 'I ran 10 blocks today' but I have read it used that way a lot in US based novels
A block is a group of houses, being bordered by roads, which means there is no set size, though long roads can have similar sized blocks of houses in them.