Trailrunner (http://www.trailrunnerx.com/), a Mac OS X solution for folks doing long distance sports, has been updated to version 3.1v541. The upgrade adds speed decoding from GPX files.
With TrailRunner you can plan workout routes on a geographical map, export the route directions as small NanoMaps onto an iPod and import workout data from the Nike + iPod sports kit back into the workout diary. Other features are an exercise plan, .Mac weblog publishing, a GPS, GPX and KML import/export, an integrated map and elevation data download from global mapping services like USGS topo maps.
TrailRunner is donationware. You can download the software, and, if you like it, make a donation to the developer.