I would take a wait and see attitude, as I know how much you want to do the brevet. Did the mt give you any reasons why you shouldn't do the 75 mile ride?
That said, two things come to mind. One, I am noticing how lack of stretching really is impacting me each year I get older. I don't take the time, unless it's been a very long ride. Hell, I haven't even been wearing the compression tights, which I *know* reduce the soreness. My quads are feeling it, too.
The second thing is that I know for me, what you are trying to accomplish would have never happened when I first started riding. I could do it now, but I still don't "train" the way you have been, just riding a lot of hills has prepared me for any events I have done. When I decide to do something, I do it, but there have been plenty of times, when in my mind, I know it's not going to happen, and i alter my goal. I did some really hard riding on my trip to Spain in May, which has lead me to give up less easily. But, I am not doing any super long or terribly difficult rides like I did there; it's like I have a pattern, which I change up a few times a year, but in the end I really don't like riding over 50 miles, not because I can't do it, but my body sort of falls apart with lots of rides over this amount. I don't want to spend my whole life in "rehab," dealing with sports injuries, etc.