I rode 60kms (ca. 40 miles?) on the weekend on this...
http://www.pacy.net/p26/index_p26.html
However I've only had it for a week! I am only mentioning it because I had a lot of the same criteria as you and this was the one I came up with in the end. It's German made, steel, heavy and rides very nicely. It actually folds very easily too, but the size and weight make it hard to carry.
Downsides: the website is terrible and it's hard to get info on the bikes. Everything's in German and you'd be hard-pressed to obtain one if you don't live in Germany or a neighbouring country!
Edit: Here's a more attractive photo of it.
http://www.pacy.net/p26/detailGaleri.../p26_weiss.htm
The other ones I considered were the Airnimal Joey and Rhino.
http://www.airnimal.eu/Rhino/index.php
There seems to be fewer options in folders for the likes of us, rather than commuter-types.
You didn't put price in either "looking for" or "less critical about". I think the really durable ones will be expensive.