Neil,
I have played three golf courses in my life that were 1-3 weeks away from hosting a U.S. Open. All of those experiences were between 25-41 years ago (Winged Foot in '74, Shinnecock in '86 and Merion in '71/'81). I was a better golfer then (although not really a "good player") and, at least in 1971, the added degree of difficulty from regular member play was not so dramatically pronounced. The same was not true of the other occurrences.
Either way, I was struck by two things: First, like you, it was incredibly difficult and not something I'd want to do very often. Second, it impressed upon me just how really, really, REALLY good those guys are. I hit a lot of good shots in all those rounds but every mistake made bogey a good score. My personal delta back then was 10+ shots and the only reason it wouldn't be any higher now is that my "base case score" has moved up considerably over the years.
In 1971, Jim Simons was obviously a factor and Lanny Wadkins was competitive. Ben Crenshaw had been in the mix the year before at Hazeltine. By 1981, amateurs were, and have remained, an afterthought on the weekend (except, it seems, at Olympic).
Good luck in October. Rather than view your round as a chore, you might think of it as a case study re: a touring pro's ability and that of, even, a top amateur. Then there's the comparison to your own game (and mine).
Also, your death march won't last more than 4 hours because your host will know that your group will be on the clock and that he (she?) will hear about it if that time is exceeded - even if you aren't holding anyone up in back of you. This is especially not easy for a first-time player but the club is committed to it for a number of good reasons, IMO. So, be prepared for having to be "in the linen" several times and take a double bogey for handicap purposes. On the other hand, savor your many successes. And next year, remember the day when you watch the tournament and be 1) respectfully in awe of the good scores being posted and 2) empathic to the very high scores that will also be posted.
Enjoy!