I too ran the Glacial 50K! I loved it. Congratulations to you!
I am new to ultras this year. I also ran the Ice Age 50 miler in May. I would definitely recommend that one for your first 50 miler. It is well organized, and a very nice course. I've also heard from experienced trail ultra runners, that is a relatively easy course - compared to other trail ultras.
I found the footing at Ice Age to be much kinder than the footing at Glacial. Ice Age is not quite as rocky. It has rocky sections, but I found it much more runnable.
Ice Age is in May, and starts and ends at the Nordic ski area near La Grange. You can find the link on this site under May Ultras. or by clicking here