Horseback riding rules. Especially if you plan to re-skill to dueling later. The pure Horseback Riding/Mobility through level 20 is one of the best early builds to stay on top of the XP leader board, though few people use it. You start early with Cowboy, add on quickly branding cattle, shoeing horses, breaking in horses, and finally catching horses by level 20. Not even trapping offers such a good early job lineup. After this though, you want to decide what you want to be when you grow up.
If you switch to pure shooting after that for example, with a few cheap clothing purchases you can hit level 30 in under a month from when you started and have the following jobs at level 30: catching horses, pony express, exploration, rafting wood, and hunting coyotes. All five jobs rated at over 40 XP. Between levels 30 and 40 with a couple of key clothing purchases (like fancyshirt and brown boots) you can keep those 5 jobs and slowly reskill everything out of horseback riding and into aim or something.
You can go to any other dueling-build routes later also, but the early XP and level boosts from jobs using riding can be well worth cost of re-skilling those 40-some riding skills later.