We do the Last Chance 1200k, and people from all over the world say it's extremely difficult. It starts north of Denver and heads to Kansas. Crest a roller, you see corn, crest a roller, you see corn, crest a roller, you see corn. Crest a roller, you see soy......
Just say no.
Another option is to cycle across Texas from Texarkana to El Paso, about 800 miles. The last 450 look something like this:
No climbing at all, but no descents either.