You have a pretty good plan. Some suggestions however.
On the posts you install consider using galvanized pipe. They make
galvanized brackets to attach the rails. These will not rust or rot. If
you don't like the pipe idea, use pressure treated lumber or redwood.
If you use lumber, be sure to cut some kind of angle on the top so water
won't stand and rot the posts. When you set the posts in cement slope the
top of the cement so water drains away.
Make sure your pickets are spaced up a bit if they stand in dirt or on the
cement walkway the ends will rot.
if you use pressure treated lumber for the posts, use stainless fasteners.
The new stuff they treat the wood with will eat anything else.
If the span length is a problem, you can add truss rods from the top of the
post to the middle of the bottom rail and then tighten the turn buckle to
take out the sag.
Use hot dip galvanized nails to attach the pickets. The ring shank or screw
type hold better than common or box nails.
For 260 feet of fence consider renting or buying a nail gun.
Make a jig to hold the picket and space it properly to save you the trouble
of placing each picket and making sure it is plumb.
For your rails bevel the top edge away from the pickets so water does not
settle in between the picket and the rail.
Select carefully the lumber you make the rails from. You don't want the
rail to be cut from the center section of the tree. The straighter the end
grain is, the less twist you will have.
About the time I had mastered getting the toothpaste back in the tube, then
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