While it's been a while (2002), I rode from Nyssa, OR to Sisters via Ironside, John Day Mitchell and Pinneville during Cycle Oregon using U.S. 26 off and on. I don't remember any shoulder issues. You can always use Google Maps Street View function to see if there are shoulders.
Heading east, at Mitchell there is a relatively short climb of a few miles. Then there is a long (over 20 miles), elevation loss to the junction with SR 19. The rest of the way to John Day isn't bad. Wish I could remember more.
Bring lots of water. It was very warm during the second week of September, and there was little or no shade much of the time.
Finally, if you are planning to ride McKenzie Pass into Sisters (worth the effort), check with ODOT first. I saw some news stories reporting that its considering not plowing this year because of the estimated cost. Apparently, there are drifts up to 30'.