Kyoto-Koyasan-Tokyo

hello. In two weeks I am going to kyoto for 4 nights. Within this timeframe a friend suggested to go for Koyasan. How long is it from Kyoto and how easy is it to get there? Can I go to tokyo straight from Koyasan?

From Kyoto to Koyasan it takes about 3 hours via Osaka:
http://www.japan-guide.com/e/e4904.html

Can I go to tokyo straight from Koyasan?

You need to transfer a few times along the way. From Shin-Osaka, take the Tokaido Shinkansen:
http://www.japan-guide.com/e/e4006.html

Dear Uji, thank you for your reply. I think that I will go to Kyoto first, then I will take train from there to Koyasan, stay there for one night and then back to Kyoto for the flea market on Sat 21. Do you suggest I buy a Kasai region JR pass in Japan? Thank you

No. The trip to Koyasan is not covered by the JR West Pass.