Park in St. Charles and do an out and back to Machens if you want to get "all" the trail. Or I guess you could park in Portage des Souix and pedal over to the trail.

And, YMMV, but I would NOT ride 94 any more than absolutely necessary- just too many fast cars, no shoulders, and places with bad sight lines/curves. Use Saale Rd and Dwiggins; no traffic...