I have an allotment of BTC that I've been holding onto since 2012. Would this be a capital gain, losing 20% at most? I mined it, so I don't really have a record.

I'd guess that you never reported the income - BTC mined - and the associated cost of mining?  If so then the "most correct" reporting at this point for the sale of the BTC would be to assign $0 basis to the transaction. 

Yes, this would be reported as a long term capital gain when you sold the BTC, subject to a statutory maximum LTCG rate of 20%, though your effective rate could be higher than that depending on your personal situation.  You would report the sale using TurboTax's "Stocks, Mutual Funds, Bonds, Other" interview.  You'd tell TurboTax that you did not receive a 1099-B and input the relevant information that's needed for all sales, however reported to you:

1. A description of what was sold, e.g.," X.XXX Bitcoin"
2. The date of the sale
3. The proceeds from the sale
4. The date acquired ("2012" would be fine even if mined before that)
5. The basis of what was sold

If you swapped the BTC for some other cryptocurrency that too would be considered a sale; you'd use the U.S. dollar spot price of the new cryptocurrency as the "proceeds."

Tom Young