My wife and I road the portion of the Prairie Spirit Trail from Ottawa, KS to Garnett, KS (out and back) and found it to be a very nice ride. The entire trail goes from Ottawa, KS to Iola, KS, for a total length of 51 miles.
Also, a HUGE +1 for the High Trestle Trail. We road from Woodward, IA to the "Oasis" and back. It was an easy 35 mile ride, along a trail paved in concrete and asphalt.