I'm thinking bicycle (road, mtn, or trainers) with ROAD being the best of the 3. It most mimics the slight lean forward of the ST. The extreme lean of of my Trek, makes riding my Honda seem feel like baby carriage. The road bike builds a solid core of ab and back muscle which are most used in motorcycling. Along the way it builds the endurance to counter the fatigue felt in arms and shoulders by leaning forward on them for long periods.
All I know is several days a week of cycling, a few months before embarking on a cross country ride, made everything about that ride better. The only thing I dislike is the inherent risk of cycling, it goes against every instinct I have as a long rider. It calls on you trust a motorist much more than you ever will riding. Depending on a car to give you room when they come up behind is not natural.