Accidental starter; advice?

I followed a recipe for oatmeal bread (possibly from here) that required a biga a few months ago. I put the extra biga in the freezer, thinking I would repeat the recipe at some point. Recently, we moved, and I packed all my freezer contents in boxes for transport, etc. This afternoon, I discovered my little ball of biga had been missed in the bottom of a box, and has been sitting at room temperature in the basement for two weeks. I noticed it was quite light, so I unwrapped it, and found it smells wonderful. I'd like to refresh it and keep it going as a sourdough starter. Does this make sense, since it was started with commercial yeast? I don't want to put a lot of work into something I'll be disappointed in. Maybe I'd be better off to use this as a kick into gear to do a proper starter from scratch?

If I do try to save it, I'm wondering if I should thin it down now when I feed it. Any suggestions would be very welcome!