I am retiring from the National Guard with 20 years of AD in the next year. Would anyone have advice on what to expect or prepare for?

1. If you plan on moving after you retire, network in your new area now.
2. Prepare to be underwhelmed at the job market.
3. Get your purely civilian friends to read over your resume and help you translate it.

There are several "What to expect when you retire" kinds of discussions on RallyPoint. Here's one with some good insight.
https://www.rallypoint.com/answers/i-will-be-retiring-within-a-year-any-advice-on-what-to-look-for-or-expect

First, congratulations on your retirement and thank you for your service. If you are planning on a follow-on career, be advised that you may encounter a tax surprised the following year if you haven't planned for it. Should you find a job that pays equal to or more than what you bring home now, your retirement check may bump you into the next tax bracket, thus discovering during your next tax filing that you may not have had enough taxes withheld to offset the increase in overall income. Also, if retirement pay for ANG is like retirement pay for Active Duty, you will need to tell DFAS how much state tax to withhold, unless it is automatic. I recommend checking on both of these issues.