Hey, EG: I ride recumbents almost exclusively. There's no reason why they should be harder on the knees than a conventional bike. For what it's worth, though I ride a high bottom bracket design (HP Velotechnik Street Machine) my knees were a bit happier with a lower bottom bracket design like the Tour Easy. I got pretty bad recumbent butt (sore ass muscles) on that one and ended up selling it. Many people don't, however, and Tour Easy's are among the best of touring machines! Get underseat panniers for sure.
In general, recumbents, when you get them stock, are geared way too high. You need low, low gears to take it easy on your knees. If you get one with a 26" rear wheel, make sure the cassette is an 11/34 and that the front chainrings go down to 24t at least. Mtb cranks--22/34/44--are great for touring, and paired with the suggested cassette will give you a fine, low gear for almost any hill.
With recumbents, when in doubt (and when not in doubt) SHIFT DOWN. Keep the pressure light and the cadence high. Because it's possible to push so hard against the seat, knee damage is a possibility if you are not careful. Also, ease into the design and give your muscles and joints time to adjust to the new position. Many new 'bent riders get too enthusiastic (I'm guilty) and injure themselves. Park the ego and take it easy--and enjoy! I would not be a long distance tourer without these bikes.
Cheers,
Scott