Input for arbitrarly large radius in map

I'm developing a mobile app where the users need to select a point in a map, and then define a radius around this point. This will be used to define a Geofence and to perform an action when later the user enters this area.

At the moment, the user can select a point by touching the map, and then sets the radius using a seekbar. The circle drawn in the map updates dynamically with the changes in the seekbar.

My question is how to allow the user to input an arbitrarily large radius?

At the moment I have defined a maximum for the Geofence radius. But I would like to allow the user to define a radius as large as they want.

One idea would be to just increase the maximum of the seekbar when the user "pushes" the seekbar on the right edge. But then when pulling it back, the scale would feel weird: small changes in the seekbar are now huge changes in the radius.