Android : Android: Right approach for sensing the distance between current location and some point of interest?

I have a question regarding my programming approach for android - whether it's efficient or not. Just to give you an idea what I try to accomplish:

There are multiple points of interests on a map. If the user gets nearer to a PIT, a notification about the distance shall appear. There are let's say 3 notifications: At a distance of 1 mile, 0.5 miles and 0.1 mile. When the user is more or less exactly at the PIT, no more notifications are shown. The user is supposed to drive a car or another vehicle.

I'm currently using Geofences to detect whether the user has entered a radius of 1 mile. If so, I check the distance of his current location to the PIT. If it's about 0.5 or 0.1 miles away, a notification is fired. Therefore, I check the current location frequently (right now every 10 seconds). If the user is like at the PIT or rather very close, the Geofence is exited and done.

The code is working fine and this approach is the first thing that came to my mind while reading the Android Developer's tutorials etc. But is there another approach that is MORE EFFICIENT?

Although this is (currently) meeting my needs, I have two major concerns:

1. 
   

   It seems to drain the battery quite a lot. Since the user is most likely to be in a car where he can charge his phone during usage, this might be less of a problem.

   
   


2. 
   

   I read the number of PITs from a file on the SD card. In my example I have just a few PITs. But the user is able to modify the source data so that he can add as many as he wants to. Google says that only 100 active Geofences are supported per user. But what if my user has more than 100 PITs? Since I'm using a Geofence to sense whether I have to fire the first notification or not - this is a severe problem. So is there an alternative?