Another site that's useful in answering these questions is the forum over at early-retirement.org. Lots of folks with checklists there.
One of the key themes there is running various retirement calculators to see how confident you can be in your projections. One question you will want to resolve if you are going to decumulate over time is what safe withdrawal rate makes you comfortable. Many researchers now suggest that 4 percent is too high and something in the low to mid-3 percent range is "safe." Determining your comfortable safe rate in those last couple of years before you RE is key to your success. You may need to adjust things to feel confident.
Expenses are also important. Have you accurately accounted for all your anticipated spending in retirement? If you are going to reduce expenses to make retirement "work," will you be satisfied with the resulting lifestyle? One suggestion is to live on your proposed retirement budget for a year or even two before you actually pull the plug.
Have a good idea what you want to do with your time. Getting away from the grind and an unpleasant work environment is one thing, figuring out how you want to spend/invest your time is another. You already do a lot of volunteering, so that's probably going to increase. List what you want to accomplish and make a plan. This was a mistake I made. I wasted a lot of time in the first few years not knowing what I really wanted to do. If I had a list of things I wanted to accomplish in the first year or two, I would have been much better off.
Set up your post retirement asset allocation. It is often suggested that you put aside one to three years of expenses in cash or other low risk accounts. This insulates you somewhat from sequence of returns risk. If you do this, you may need to spend some time reallocating.
Establish any lines of credit you may need in retirement. Stabilize your mortgage situation, get a HELOC, apply for credit cards, and so on. As I have mentioned elsewhere, banks do not like to see "retired" in the employment box on a credit application. There are fairly rigid requirements in place now for banks to determine your ability to repay debt. Social Security and pension income are counted. Most lenders either do not count or heavily discount asset based income. They may make an exception for rental income, as long as it is stable and has appeared on your last two tax returns. It depends on the lender. If you are liquidating paper assets to eat, you may have serious issues getting credit.
Health insurance and estimating your SSA benefits have already been mentioned. Understand that you are anticipating a long retirement, so these are guesses at best. I was 9 years away from the first possible SSA check when I retired. I looked into it to make sure the earnings were correct, and I estimated the benefit. I did not need the money, so I considered it a nice annuity that could be used for enhancing lifestyle or could be invested when it appeared. If the annuity is important in your calculations, consider various scenarios and integrate them into your calculations as you approach RE.
Finally, you can try to negotiate your separation from work so that you get some extra money. The Financial Samurai negotiated a layoff with severance. He was so successful, he wrote an e-book about the process.
