There is a much easier way to do this.
(1) First, make sure you’re signed in to Google with the account that has the feed in Google Reader.
(2) Hover your cursor over the subscription to get the feed address (even if the address doesn’t exist anymore). For example:
(3) Change “view” to “atom” and change “?hl=en” to “?n=1000”. For example:
(4) Run the modified URL in your browser to get an XML file with the last 1,000 entries from the feed. Save the file to disk.