Almost any working farm breed of dog would make a good trail dog, as long as they are completely trained to NOT chase any other animals. Think about it...a farm dog may work an 8-hour day, then spent the next 16 hours in a crate, so lots of down time is not a problem as long as they get a good run every day.
Otherwise, any lower energy dog will work too. The important thing is that, come weekends, you get them out there putting in the miles, building up stamina etc...but avoid high impact agility type exercise until their bones are fully formed. If starting with a pup (recommended), try to expose them to as many new experiences as you can early on. Things like children, people wearing funny hats, people with packs on, cars, boats, bicycles, skateboards, rain, snow, ice, other pets, livestock, tents, river crossings, swingbridges, etc...make them feel entirely comfortable in these situations.