I participated in a marathon race...

I participated in a marathon race last December held in Nara city last December.
Running this race was much harder than I expected!
You know, The course had too much many ups and downs to run to the end.

Good job! I don't think that the sentence "I participated in a marathon race last December held in Nara city" is wrong, but "I participated in a marathon race held in Nara city last December" sounds more natural to me. "Last December I participated in a marathon race held in Nara city" also sounds better to me!

"You know..." also doesn't sound like natural English to me.

Sounds like it was a fun race!