Thanks for your replies, everyone. You're right, we need a plan.
Maybe you guys can help me figure out a way to start. There are so many unknowns. I am in a troubled industry, so I'm not sure what the future will hold job-wise. I may have to change careers or become and independent contractor eventually. My wife is in an OK industry but it's still the cold hard private sector, same as me, so who knows what the future holds there. We are not high earners in the first place: Between the two of us we have gross income of $80,000 to $90,000 per year. Furthermore, we do not have great potential (as far as I can see) for promotions, bonuses or earnings leaps.
As for other sources of uncertainty, well, I hope my kids get scholarships -- that would put us in a much better position. But that's a long way down the road.
In a way I hesitated to join and post on an early retirement forum, because I'm not sure how realistic it is for us. But as I said, I am still "hoping" for an early semi-retirement. Our saving graces may be that we are not materialistic, we live in a fairly low-cost area, and we're open to moving to an even lower cost-of-living country eventually, at least for a while.