Maybe he would like to train for a marathon in your area and you could be the "trainer". You could ride your bike and encourage him, time him, know the course, carry the water, etc. Then, for the actual event you could volunteer. Lots of hospitals and community organizations sponsor 5K's, you could get involved in running, be supportive, of his hobby without actually having to run? I don't think running is really a "together" thing maybe take up camping and hiking?