Exact number is 25m ( See “Making them count” @ About the Node, Street, and City Data ).
For me, I have done both methods above, run both sides, towards oncoming traffic and walked medians (they are usually overgrown, and I don’t trust running where I can’t see).
One thing I didn’t see mentioned, is you only need 90% of the nodes to “complete” the street. depending how many total nodes are on 42st St, running one side of that median may be enough.
But I have seen many people who want “all the nodes to be green”, and lines on every street of the life map. In fact, that is my goal too.