Nice work on staying consistent with the ME and I’m glad you’ve seen solid results – it is a real weapon!
As far as tapering off the protocol in advance of your 50-miler, I’d suggest stopping the ME gym workout 3-4 weeks in advance, and in your final weeks (maybe two) before you fully taper all training, you can do some more specific ME training such as harder uphill repeats and/or some fast, sustained downhill running, both of which will push that M.E. system you’ve developed in a very specific manner.
We’ve seen in many athletes, and i’ve observed in my own training that the effects of that ME protocol will last for many weeks following cessation of the workout. That means you can take the gains you make in the workout itself and apply them very productively to more specific training in order to boost economy.
Good luck; I’m keeping my fingers crossed for all of us that these late summer races are able to happen!