Apple's yearly developer conference should see iOS 8, OS X 10.10, and likely some hardware.
Apple Exercise iPod Tempo Patents
The Nike Sport Kit incorporates an accelerometer placed in your shoe that communicates wirelessly with your iPod nano. The iPod nano then displays additional information about your exercise including time, distance, calories burned and pace.
The patent application reveals that Apple has also been working on additional features that could sync your exercise to your music. One possibility described is syncing your music to the same tempo as your run.
The Pat App suggests either changing the tempo of the song or the iPod selecting songs which match the tempo (the host computer would analyse songs and add a tempo tag to each).
One aspect of the patent is to use your running speed to set the music tempo- if you're running slowly it'll play slower music- running faster- faster tempo music.
Tempos could be adjusted based on user preference, but sample menus also reveal a "Training" option which could offer different tempo profiles, increasing and decreasing over time.