Hey Evan, I could always lean out parts of my body as needed by specific training. I have big honking quads as well.
When I was wanting to do some hard rockclimbing route that required a lot of upper body strength and smaller legs, I ran ONLY flat. No hills at all to lengthen the muscle instead of bulk it. No hard climbing on the bike or anything, either.
However, if your thighs are wanting to be muscular, it is hard to maintain without specific training like that.
When I was your age, I changed my "shape" at will. Obviously the natural shape of your body is there but you can refine it.
You don't want to lose muscle mass, get rid of the body fat before all.
So to summarize. Don't eat a bunch of crap, and run on the flat.
Now I'm ancient it takes a little longer, but that's a whole other story.

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