I never flew the F-111, but in talking with some of the crews, the later models would keep accelerating until they melted. I think they had a light that came on that said "hot" or "slow down" or something to that affect. I flew the F-105 and had it up to 825 to 850 (memory is slipping) which was the limit as far as the book said, I am sure it has seen faster. I was going downhill in full burner, not level.
While based in Europe back in the very early 1970's, I was driving an F4E and spotted an F-111 (don't know which model it was). We were down around 500 feet or so. I pulled up on its wing and he began to bump it up. I thought I was hanging in fairly well when he bumped it up again. I lit the burners and hung in there, and then he lit the burners and just became a dot disappearing on the horizon.