Cross bikes are actually different from adventure/gravel bikes. Cross bikes are designed for racing and generally have more road bike geometry. Gravel/adventure bikes are more like touring bikes in there more laid back geometry including longer chainstays and they have the ability to take really big tyres, in the main, a true gravel bike would be ineligible for CX races.
As for distance, well I've toured off-road on several long distance tours, Slovenia last year was my biggest, over 440km and 8000m of ascent, including the wonderful descent of Mount Stol:
I've popped several travelogues into the touring section of my various off-road adventures if you are bored!
Germany has thousands of kilometres of forest tracks, where I live is very sandy and loose soil so it's the ideal domain for a good fat tired gravel bike. I currently run a Kona Sutra LTD with 44mm tires, disc brakes and 1x11 groupset. It's an ideal multi purpose bike and a super comfy off-road touring machine.