Riverfront Trail- runs north to Chain of Rocks Bridge, then you can cross over into illinois and continue on trails to Pere MArquette park. Round trip if you go al the way to the park and back is ~ 100 miles. Beautiful ride along the river north of Alton this time of year. You can also ride from downtown to Forest Park along streets, most with designated bike lane/wide shoulder. Again, stlbiking forum has the experts.