Map-reduce it with overlapping sections, but then propagate back downwards and repeat the process, but now each map-reduce node knows the context it's operating in and can summarize more salient details.

Concretely, on the first pass, your leaf nodes are given a prompt like "The following is lines X-Y of a Z length article. Output a 1 paragraph summary."

You then summarize those summaries, etc. But then you can propagate that info back down for a second pass, so in the second pass, your leaf nodes are given a prompt like "The following is lines X-Y of a Z length article. The article is about <topic>. The section before line X is about <subtopic>. The section after Y is about <subtopic>. Output a 1 paragraph summary that covers details most relevant to this article in the surrounding context."