If you started a business after retirement, how far in advance did you begin preparing (business plan, loans, etc) and any advice for others?

Two words for you SFC, "Business Plan". I owned a successful business that I sold after 4 years (my CO's decision). One of the things I observed of other small business owners is they started a business based on their interests or area of expertise, but they didn't have a clue about the business side of their business (i.e. accounting, taxes, marketing, target market, etc). If you don't know where you want to go then you'll never where you'll end up or fail to plan, plan to fail, or whatever cliché you want to insert. Two things about your business plan, 1) it's never too soon to start; 2) it is a breathing document, always changing, so keep it updated.
One other word I have for you, "Networking". Do it! It help me grow my business by 40%. If you don't have a bachelors degree, get it. It will help with loans and investors.

I started 2 different businesses, 1 - a genealogy research business, the other a advertising/publishing one. The both did fairly well, for the first few years, then the economy took a hit, and so did mine and so many other small businesses. I still from time to time do projects (genealogy) for friends, but I was forced to close them both down. Both were planned starting about 2 mos after I left service, and were ran out of my home, so the overhead was pretty low. Time, money, interest in the job, and ability to actually perform the job, organizational memberships, etc., are all tings to consider when starting a new business. Hope this makes sense.

Business plan help you build a road map but it is important that you figure out what it takes to be profitable.

How many monthly users it takes to pay all the bills. Lease, lights, equipment etc.

Next figure out what you want to earn and if you will need partners.

Send me a PM. All the best.