Once you get the sponsors, the important payback is to share the experience. People pay to follow along on your trip and learn something; perhaps even get empowered in their own lives. The world need role models and you get paid to be one.
Today's communication technology obliges well in sponsor aspects. You have the chance to write honest accounts of your journey, snap pics and shoot video to feed back on a continuous, instant basis. Your adventure can reach people on their home computers, or their mobiles whilst on a lunch break at work, or just anywhere and anytime in their lives. This kind of feedback from your expedition creates great interest for you. People feel like they know you and will want to come back to watch your progress.
A business plan for pay-back should include pics, video and dispatches that the company can use for tradeshows, internal campaigns and customer campaigns. A Media plan should include how many articles with your sponsors name in them that you count on raising. And finally a web plan should give easy access to your expedition with your sponsor's banner ads well exposed to much traffic.
In the end though, it will all boil down to if the people you pitch like you and the spirit of your project. If they do, they will want to bring you in to empower the company staff and they will want to be associated with you in media. That is the essence of image making that all companies work with. Athletes are signed in promotional campaigns because they represent strength, endurance, success and integrity. Hanging out with a guy like that rubs off on the company image.
You don't have to reach your goal for the sponsors to be happy. “Rocky” movies always begin with a few lost rounds. A fight against all odds makes the final success much more interesting. Your sponsors might well come back with you year after year, provided that you deliver what you promised in terms of pictures, video and speeches and provided you really fought the battle well. Again, it's the ride that they are in for, not just the destination.