Best kind of bicycle for lower back issues? I've ridden bikes all my life.  Now nearing 50, for the past year or two I've noticed that when I ride over 20km or so, my lower back gets extremely painful - enough to make it not possible to ride.  The bike is the same one I've already had, so it's not a bike-fit issue.  

Because the bike is breaking down, I need to replace it.  I'm worried about getting a bike that I won't be able to ride for anything other than short commutes because of back pain...and not finding this out until the first 20km+ ride.    A short test ride of a new bike isn't going to be enough to determine whether it will trigger pain.  Knowing that it's the lower back, what style of bike should I consider?  I currently have a mountain bike and am thinking either of a touring bike or hybrid.  I probably don't have storage space for a recumbent.

Oh, and current bike is a mountain bike.   In the past I did heaps of touring on it, with no issues.  Now I use it for commuting, and sub-20km rides.
I have a touring bike and I find it quite comfortable. But them again, I don't have back issues. It's a bit more upright than most road bikes, and possibly more upright than some hybrids I've seen, as long as I'm riding on the hoods. Try to find a place that rents bikes or will let you have an extended trial ride in order to determine if it would help fix your issues. Spending $100 renting a few different models for a few hours at a time could save you a lot of money and help you find a bike that's comfortable for you.