Agree with O4U, when Raggy and I went on our mandate last year we walked everywhere too - it was February, so brisk but not bitter - once you get moving you are fine. Despite the block-system (which genuinely is super-easy to navigate) we did get lost once. But it was 1am and we were smashed, so that's on us.
I think from memory, because we didn't want a travel card, we paid about $15-20 for a single in to town from JFK - Raggy may remember better than I do, he usually does.
Oh, and get the app citymapper for whilst you are there (requires data/wifi) if you are using the subway/busses, it's brilliant.