I haven't read the rest of the replies, so I may repeat some of what has been said.

I live in a windy area and have dealt with this often.

Getting down in the drops can help a lot. Anything you can do to be more aerodynamic. Keep your elbows in. Wear tight fitting clothing. Drafting is HUGE! Any time you can do that is wonderful in wind. You said you did that and were still struggling. I think in that instance, the fact that it was your first 50 miler, having only done 24. That's a big leap all by itself, and the wind only compounded it . . . by a ton.

If ever you can draft behind two or more people, and if those people are side by side, that creates a bigger draft. Make sure you are close enough to the wheel in front of you to get the best benefit of the draft. Also, try to find that sweet spot. If the wind is coming at an angle slightly from the left, then being slightly to the right of rider in front of you will be the better draft.

However, sometimes you have to ride in the wind alone and sometimes no matter what you do, even with a group, it just plain sucks!!! That's when you remember that while your speed is slower than you'd like and it hurts and is frustrating . . . it's making you stronger. Some day when you are doing a ride with zero wind resistance and you are flying even faster than you used to, you can thank some of those nasty windy rides for your especially great ride.