WE are planning to go to japan in September. is it too hot to hike in the Kyoto area in sept?

Is 82F too hot for you? Might be wet.
http://www.weatherbase.com/weather/w...me=Kyoto-Japan

Liable to be humid when not wet. Can you wait until October?

I am going to echo thursday's sentiments. I have tried hiking a couple of times in September in Japan, and I aborted the mission because of the humidity--it's extremely high in Japan during certain months. It really wears me down. I'm not sure if it's because I'm from Southern California, where the air is dry, or if it would affect everyone. In any case, I agree with thursdays that you might be better off hiking in October.

For historic climate data, you might want to look at
https://www.wunderground.com

Temperatures in Koyasan are five degrees Celsius lower than those in Kyoto.