I don't believe it's possible to retire early, or even on time, with just a 401k, if you want a comfortable lifestyle that isn't a miserly lifestyle.
You can only contribute $15k - $18k (over the course of last few years) into a 401k pre-tax. So unless you're doing after-tax funding (which I have done in the past), your 401k can only grow so much given the limited contribution.
You really need Roth IRA's, IRA's, and other investment vehicles to boost that nest egg.
If all you have is a 401k, you're probably better off than most people. But if you have a 401k, IRA, Roth IRA, post-tax investments, you'd be in far better shape.
The problem is most people don't even contribute to the 401k. Their greatest asset is that shiny new depreciating cars sitting on the driveway, and maybe the house that they're paying on.
To retire early, means you need investments to draw on. You can't draw on a 401k until retirement age, so RE is out of the question with just a 401k.