I have no idea how App Engine is suposed to be structured, but that error is nothing to do with conversions from String or anything.
It simply cannot find the method because the class with the getDistance() that returns Float has been replaced by the one that returns String after the server has generated the JSP code based on the version that returns Float.
I don't know the server your are using, so I don't know how you get it to regenerate and compile the JSP code.