we've done a few challenging travel ones - check out some of the ones on skedaddle.co.uk. we did their Colombia road trip at the end of 2011 - we were on the first organised bike trip in the country - that was bloody tough!! long climbs, some pretty steep but balanced by long descents. they've changed the itinerary now to make it a little easier but it will still be hard work.
MTB'ing in Peru was tough due to altitude - some days over 4000m on the Altiplano.
Spiceroads tend to be more tourist than challenge although some of theirs are tough as well - check out the Kazakhstan MTB one.....
we've got to the stage where perhaps the trip is more important than the severity - seeing a country from a bike is a great way to travel
PM me if you need any help - we're pretty seasoned bike travellers now....
