New in the Nokia Store is Sport Timer, a simple but effective 'interval timer' for runners. Backed up with headphones for the audio alerts, this can help you in accurately tracking your run/rest intervals. You can set up multiple interval profiles and then tap to select and start. If you get interrupted, just tap to pause and resume, etc. Some screenshots below.

In case you're not up to speed (see what I did there?...!) on interval training, Wikipedia defines it as:

Interval training is a type of discontinuous physical training that involves a series of low to high-intensity exercise workouts interspersed with rest or relief periods. The high-intensity periods are typically at or close to anaerobic exercise, while the recovery periods may involve either complete rest or activity of lower intensity.

Here's Sport Timer in action:

A default profile is there to start you off and show you how things work, tapping the main button sounds a klaxon and you're off...

Various languages and volume/alert options are available; setting up a new interval profile (right)

A nicely implemented (in Qt) free utility, Sport Timer does one thing and does it well. Grab it here in the Nokia Store.