I admit, we didn't make it to Lake Hangetsu as we were not prepared for the trek. There's hardly any reviews here for visiting during summer so here it is:
This is a proper trek and not a walk in the park. Proper trekking shoes are recommended, and not sneakers, which we had on, as the path can get slippery and trees/shrubs can get in the way. We saw a Japanese couple come fully geared for the trek as we exited the path, and they probably thought we were nuts to attempt it without proper shoes at the very least. However, we did come across a tanuki (raccoon dog) from a distance, so I was a happy camper even if I didn't make it to the lake.