I'd do what I do pretty much any time I have old gasoline to dispose of-mix it a few gallons a time in with a full tank in a modern FI vehicle. A modern car won't know the difference, and it will save you a few bucks on gas in the short term.
The last time I had to do this, I'd pumped the tank on my MG after it had phase separated, although it was ~12 gallons and not 50. After I'd separated the water layer off, which also had most of the alcohol, I was left with gas that I'd be afraid to guess the octane of, but I imagine it was in the dumpster(since ethanol contributes a lot to the octane rating of modern gas). I'd put 1-2 gallons in the MKZ before going and filling up with gas, and it didn't miss a beat.