My beach cruiser is a 7 speed Electra townie with fat tires - have gone on 30-40 km rides with; however not sure the saddle is optimal for longer tides.
Ps. The Townie's gearing for longer rides, while more effective than a single speed cruiser, could be improved. Stock gearing is the "mega range" cassette with a huge jump from 24 to 34 teeth- a cassette with teeth counts more evenly spaced over the range of cassette would be better.. In the 7 sod set up, a 40 t cassette and the 13-34 cassette would be more effective.