Physical activity apps have been receiving much research attention because of their potential to promote healthier lifestyles. However, despite established benefits from being physically active and the potential for new activity apps, global activity levels are still low. Many gamified systems have been created to increase enjoyment during exercise, yet most focus on single-player interactions and do not enhance long-term engagement. In this paper, we present our initial design and findings of KosmoWalker, a social space-themed, step-based mobile game that turns walking into cooperative missions by converting real-world step counts into virtual points that move teams through space via goal setting and team challenges. Results showed that the prototype is engaging and persuasive, particularly the experience of participating in competitive challenges and shared goals, and that the user interface is usable and easy to navigate. Overall, these findings illustrate the promise of social and collaborative exergame design and inform the development of a functional version for broader evaluation.