In 2000, I joked to DW that if the 401k/403bs continued on the current path, that we might be able to retire at 55ish--then Enron hit (she was working at Dynegy and her "company 401k match in Dynegy stock" went down to almost nothing, although her contributions did fine. At that point, you could only take the match in stock, although the law changed on this post-Enron).
However, I did a retirement plan in Microsoft Money at that point and plotted out a relatively frugal retirement at 55, 60, and 66, just for kicks. After dodging the bullet of the tech crash in the early 2000s and then escaping comparatively unharmed from 2008/9, I realized 60 or 65 was quite possible, although the market would need to "cooperate," which it did, in spades.
About 20 months ago at 57, I went into "Voluntary Modification of employment" (working less than 1/2 time online for 1/2 salary) and never looked back. DW, who is 5 years younger, may stop next year or in two years at the most. I can ski or fly fish in the morning and work in the afternoon or evening, or vice versa; I like the freedom of working on my own schedule and haven't touched retirement savings and won't for the next two-three years. Withdrawals then will fund from 62 to full-retirement age (or later, if the market cooperates; earlier, if not).
We now have 70% more assets than I originally calculated we needed and consequently am planning a less frugal retirement, when we start to draw down in a few years.
As Midpack indicates, Four Pillars of Investing is a fine resource.
And on frugality, reading Thoreau's Walden in high school, particularly the first two chapters, cemented the idea of work/expenses versus "experience"/free time, an idea I never forgot.
My Okie grandfather retired a few years early after building his own cabin in Colorado about 8 years before he retired (he was a rural postman), so that also was a powerful example. Going up there in summers in my youth to fly-fish made a "yuge" impression. Paradise.
I believed in a flexible approach, adapting multiple plans (a matrix) to what might occur, and still do. DW originally wanted to work for another 6 years, but I'm trying to convince her that she can stop when she wants to do so, including next month. I'm getting renewed for at least two years, so that makes this more than a possibility.