Can you show the leaves and flowers? It is certainly a Brassica of some kind. Quite possibly a turnip as you suggest. Was the area ever a vegetable garden?

I'm afraid I hacked at the leaves and flowers before I pulled it out, but I'll go out and take a photo of what's left. Yes, it was a vegetable garden. In the area I found it it was planted with salad greens, most recently buckwheat to replenish the soil.

Inside the root.

I think I'd plump for a prehistoric turnip.

Ha, perhaps! whatever it is, it grew incredibly quickly in just a few weeks, so perhaps mutant turnip.