I worked for BigCo and enjoyed the industry and work but not the corporate environment. I was contemplating buying a small business where I could run the show without all the bureaucracy. It's tough to cut the golden handcuffs, so I dithered. I was blessed by a job loss that was the exact kick in the butt I needed to launch in the direction I wanted.
Barring that, I guess the main ducks you need to have lined up are:
1. Decide what your biz is.
2. Make sure you've got the funds to do it.
On #1, it doesn't sound like you've decided what your encore career is, you're just itching to get out. You should do some soul-searching and figure that out. Do you love what you do, just not the environment (like me?). Or do you hate what you do and want to launch into something fulfilling (like MMM)?
On #2, you say you've saved up, but what does that mean? Are you at the FI tipping point? Or do you just have a couple years of living expenses saved up to cover the potential risk?
The more money you have, the earlier you can jump without having everything figured out. One approach is to start doing the new thing on the side and when you're to the point where you're making half of what you need on a PT basis, you ditch the FT gig and make the side job FT. That can work whether it's in the same line of business or something completely new.