Yeah the pidgey evolving seems to be the most efficient way. I farmed up 300 candies then popped an egg and just started evolving. It's a boring 30 mins but gets you a lot of xp fast.
I would also suggest waiting until you are at least well into the 20s before evolving your magikarp. I made this mistake early and now it costs a hell of a lot of stardust to increase CP every time I lvl up.