My experience growing horehound from seed was a long process. I planted the seeds more than once before they actually germinated, and then they sprouted at least six months after sowing. Unfortunately, I didn't keep good enough records to know the details but, I know I had planted over the horehound with some other seeds before the horehound showed up!
It's been growing for about three years now, just the one plant in part shade shows back up. It has never spread or become agressive.
I think it can be known to have irratic germination. Wildflowers are often like that too.
And, herbs seem to act different in various climates.
I've had the same experience with comfrey ~ everything I was reading said how agressive the plant is, but not for me, I can barely keep it growing. Come to think of it, it's the same with catnip. Now, everyone else can grow that, except me!!
As of today, I can see my little horehound plant hiding beneath the leaves, waiting for spring. Once it wakes up this spring, I'll be happy to share a piece with any of you up there.