I'm doing some similar searching for the perfect all-around bike, and I've narrowed it down to either the Salsa Fargo or Surly Ogre. Both are built like "29r's" (wheels about the same size as 700's) but seem to be adaptable to both road and light off-road use (we've got graded dirt fire-roads/fire-breaks here in the Southern California backcountry that are fun to ride and don't really require a true MTB). They have disc brakes (the Surly can also use rim brakes), and both can use skinnier tires. Leaning towards the Surly since it is more adaptable and can accept racks and an internally geared hub for the rear wheel. I'm not too interested in the fastest/lightest bike, but one that suits my needs and (like you're trying to find) comfortable for longer rides.