Barb suggested the original idea. The script searches a user's Twitter feed for tweets tagged with '#bike' or '#run' or '#walk'. Each tweet is scanned for a distance in the form 'N miles', 'N.N miles', 'N meters', or 'N km', and for a time in the form 'N.N hours' or 'N minutes'. The scanning script tries to be polite in its scanning of Twitter, caching tweets in an SQLite database and only re-scanning at intervals.
The HTML output of the script uses jQuery to assemble output based upon the stored data. Currently the final output is very simple, just a table of dates and the exercise activity on that day. This could be made much fancier; the script could output either HTML or JSON, Imperial or metric units; there could be different tabs to view bike rides, runs, or walks; graphs could be produced; etc. The goal is that you could put an IFRAME in your weblog's sidebar and show a running total of your exercise.
Currently I have the CGI script up at http://www.amk.ca/cgi-bin/running.cgi?twitter-id. I don't promise to keep that URL alive indefinitely -- eventually I'll need to wrap it in a little Django app to let you customize your display -- but you can play with it for now. The Mercurial repository is at code.amk.ca/running-sum/.