Sport Comfort Bikes

[BleeckerSt_Girl]

Most touring bikes are able to take wider tires (wider than 23c) to navigate on varied less-than-ideal surfaces. That means wider forks and wider brake reach.
They also have braze-ons & eyelets that you can attach racks, bags, and panniers to.
Their frames are made to carry the extra load of camping equipment.
They have the ability or put two or three water bottle cages.
Most have drop bars rather than straight handlebars, because over many hours of riding you really NEED to change your hand positions often.
They often have a longer base or longer trail which makes them more stable in handling and less 'fickle'.
They usually have a wider gearing range than typical road bikes and can climb mountains better.
A lot of people do like steel bikes for touring because they give a pretty comfortable ride. The least desirable frame for long distance would I think be aluminum.

You don't say what you mean by 'serious distance'....30 miles? 130 miles?