I'm thinking of quitting a volunteer position after completing the training
Why would you quit your volunteer position?
It has been too long of a time since the training.
It is fine change your mind about volunteer work. Donate as much or little time as you feel like.
I'm going to notify my boss I quit and let him try to get someone else to take it! Hah!
It's just a volunteer position, so there shouldn't be much harm done! You were only figuring out what was best for you, and it's great you found out now before you no longer enjoy things.