I was there about 2 weeks ago. Depending on when you go, be ready for heat and humidity. None of the hills that I rode were so long that I couldn't pull them, but don't be afraid to stop and rest. I expect most of the amber waves of grain will be harvested by now.
I found that the flint hills area was beautiful to ride in and because of the rolling nature, you have a natural wind break alot of the time.
Have fun and good luck.
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TroyM - bike riding, guitar playing, retired high school agriculture teacher and regular guy.