From Sven Schmidt:
Note: It might not be obvious but the distance list on the application's home screen can be scrolled to reveal more preconfigured distances. A future version will improve the display and also allow adding custom distances.
Pacemaker is a very simple application to compute your run time for a range of standard medium to long distance runs. Simply use the dial to set your expected pace and the times will update to show your finishing time.
When you tap on a distance, a detail view will open and show you a projection for other distances from the distance you chose. For example, if you run 10km in 45 minutes, you can expect to run a half marathon in 1:40:11. This is not simply scaled by by a factor 2.1 (the distance factor) by accounts for the fact that you'll be somewhat slower over the longer distance.
The defaut pace units are "minutes per km". This can be changed to "minutes per mile" in the iPhone settings application. (This is the application where you also change your network and sound settings for example). Open this application and scroll down until you see the Pacemaker icon. Click on it and choose your units.