Additional Info (Unusual EC)

I do an extracurricular that I feel many people aren’t familiar with. (I.e. they would have misconceptions about it, if not a complete misunderstanding.) Sorry, I am not publicly disclosing specifics in order to maintain privacy.
I believe it could be misconstrued as a frivolous activity, and as such want to use “additional information” on the Common App to explain the effort/dedication it requires and why it is so important to me.

Should I write about what work I put into it? I listed it as an activity (with a brief desc.) and put the hours per week spent, but I doubt that helps with someone understanding what I actually do for the activity. And if I do talk about this, how should I do so? Should I speak objectively (maybe even use a quote from an organization, so it’s not like I’m just extolling my virtues) to describe it? I don’t know how I would detail the obligations of the activity without making my description sound self-satisfied.

My other question is whether or not to include the personal aspects of the activity’s significance to me. Objectively, it’s interesting (at least I think so, but possible bias…). But it really has been one of the most important things in my life, and I find it difficult to describe that. I also realize I shouldn’t clutter additional info and waste anyone’s time, though.

I am expecting some people to say that I should write about this activity in an essay instead. I seriously considered that, but ultimately I don’t believe the activity defines me and chose to write a more mundane, but revealing, essay that is based more on personality than topic.
But, with this essay issue, a few of the schools I am applying to have a separate supplemental “Describe a significant EC”. How do I go about that if I am writing about this activity in additional info? Would I write that on a different activity, or duplicate?