This week, work with the 500 meter thing. Keep your workout 30 mins long though. Walk for 5 mins and use your nike+ if you want, to know how far 0,5 km is. Slow down the pace but keep running. If you're a beginner, running 500 meters might take you up to 5 mins. It's a good thing to pick a song of about that length so you can tell yourself : I'll run til this song is over.
Deeper and deeper with Madonna worked for me. It's 5 mins and 33 seconds.