I'm only in my early 20s, but I've had self-diagnosed plantar fibromas for years. Two odd, permanent bulges on the arch of my right foot. I probably noticed them when I was 16. They're only painful or even noticeable if I really press on them, and they haven't affected my running at all. I've never needed to seriously treat them. My understanding is that they're commonly asymptomatic like this. It sounds like yours is more painful. Are you sure it's not just plantar fasciitis?
The only treatment I'm aware of for plantar fibromas is surgical removal, which would probably not be advised unless it causes regular pain. Other than that, probably the only thing you could do is try new shoes/orthotics to shift weight away from it.