You'll never drill in to a concrete fence post.
I did, to a point anyway. I had five to do; just a hole top and bottom.
Did the first two, top and bottom (8mm drill to get a plastic raw plug in). Flipping hard work and stop/starting to let the drill and drill bit cool down.
Then did the tops of the next three; I could really lean into the drill because of the height they were; but the bottoms were impossible.
Luckily enough there was a big hole all the way through (that the coach bolts would've went through), so I whittled some wooden raw plugs with a knife and some rough wood and bashed them into the hole with a hammer and used them to secure the wood to.
I just had to saw off the excess wood sticking out of the hole.
Still not finished though.