I played a big buyin event a few years ago, about $4500 entry (it was in euros), 600+ players. Doyle and Todd Brunson were the big US names, and the field also included most of the top Europeans.
I busted a few minutes before the end of day 1b, so got to play for nearly 8 hours. During this time, I saw about 17-18 players at my table (I never moved), and was pleasantly surprised to find that only one (a former WPT winner who was on my left fortunately) was obviously way out of my league. I thought two or three others were better than me, but to a smaller degree, one seemed weaker, and the rest were either around my level.
From this experience plus watching parts of days 1a, 2 and 3 from the rail plus talking with various players, one of whom later made a WPT final table, I'd suggest a couple of things. One is that unless you draw a tough table, you won't really be out of your depth on day 1, so don't feel intimidated and don't go in expecting to be overwhelmed.
The second is to know what adaptations you want to make to your normal game because the stacks will probably be deeper than you're used to online.