First of all, Amanosato is 30 km from Koyasan and really only reachable by car. Koyasan is reachable by bus. I did see a local line (#21 was the number) that a lot of folks were waiting for. Possibly it goes to a train station. The town itself is too long to walk. The two most famous sites are a temple that is the HQ of a major Buddhist sect and the funeral mortuary of its founder. They are at least 6 km apart. I really think you need a rental car here too unless you take an organized tourist bus trip (which will be with 30+ other people).