Exercise video games have become increasing popular due to their promise to increase fitness and reduce obesity levels, and due to the emergence of cheap interface devices. Previous research reported mixed results about the effectiveness of such games. Common problems are the lack of long term motivation of users, games not taking into account specific patient requirements, repetitive gameplay, and vendor lock-in. In this paper we design a novel exergame which addresses some of these shortcomings. The game employs an infinite randomly generated game environment, uses immersive technologies, and can be customized to take into account patient requirements. We present a prototype of this game design and evaluate its effectiveness using different levels of immersion. Our user study demonstrates a small but statistically significant increase in exercise performance and motivation when using the exergame. Employing the Oculus Rift resulted in a slightly higher motivation, but no noticeable change in performance. The head mounted display was most effective for sedentary users.