Looking for something more than iRun? Then iRunPro is your perfect application. Same ease of use, plus step pace graph, and acoustic alarms!
Just start, and you'll see the graph of your pace over the last 5 minutes. Also, double-tap on the graph, and you can set the minimum pace alarm by simply scrolling your finger up/down. Double-tap again and you'll set the maximum alarm. Double-tap again and you're back, ready to start. Setups are stored for the next run, of course.
Whenever your pace falls outside the min/max range an alarm (low or high) warns you. And every 100 and 1000 m a short, gentle sound informs you, too.