Oh yes, do NOT use pine for run posts. And remember it is not at *all* trivial to replace them -- you have to detach the wire fencing everywhere, then pull the posts (getting all the rotted bits out of the bottom of the hole, then set the new posts in the old holes which will not work as well as it did the first time b/c it'll be much harder to get the post firmly set, then reattach all the wire so it is still snug, which it won't be because the posts will be in slighly different positions). You do NOT wanna do that
And there is nothing you can paint on to them that will seriously extend their lifespan. There is a reason that pressure-treated wood is PRESSURE treated. Just surface treating will not do the trick. Even old-timey farmer 'recipes' for treating posts involved SOAKING the posts in nasty chemicals for months.
I would strongly suggest biting the bullet and buying pressure-treated 4x4s, or at least decent-diameter cedar fenceposts (nothing smaller than 4" diam.).
How far apart to set the posts depends on how your run will be constructed; if you will have both top and bottom horizontal wooden supports, in addition to the posts themselves, you can go a bit further than if you didn't. As a generic recommendation (remembering that the exact details of your planned run may change this...), I would say 5 ft is the most sensible spacing, you can go to 6' if it is mathematically necessary but beyond that gets a bit iffy, especially if you are not experienced at pulling wire tight and/or are not using a top rail.
Make sure to set the posts deep enough in the ground, at *least* 2.5' and preferably deeper, and tamp the fill dirt *very hard* every 4-6" as you refill the hole around the post.
Good luck, have fun,
Pat