Of course there are 'VR trainers' out there, but they cost a fortune and, of course, their video is junk and I doubt that anyone will have Rift for providing immersion any time soon.
Treadmill perhaps the easiest - provided you can read the speed and heart rate wirelessly from it. Not sure you need to 'steer' as such - just emulating a jog on a preset (yet interesting) landscape should be fine, I guess. Still much more interesting.
Cycling is tricker - you need to know speed, cadence, heart rate, and your steering (can be optional if the program would follow preset course like above, but this is much less interesting this way). You would also need a bike (but ANY would do) and a trainer - again, you can get a cheap one (like this one: http://www.chainreactioncycles.com/Mode ... elID=74811), and somehow provide a set of detectors - or tap onto one of VR ones, ditch stock software and add Rift functionality on top.
First way is much cheaper, and not hard at all - at least as much as I can understand.
Second would provide a way to automatically vary resistance based on information about current incline and road condition at the very least.
But of course the core of it would be the immersion provided by the rift. It would not replace true mountain biking (trailriding, allmountain, freeride), but road biking and touring simulation would spice up training immensely, and would give a stimulus to do it in RL for couch potatoes - because, unlike guitar hero - you CAN transfer much your effort into RL performance
. (Again - 'technical riding' disciplines excluded).While I'm quite poor by European standard (being a mere sysadmin from Russia) I would pay some very good money for a quality simulation. Our parts (center Russia, all plains) are not very rich in scenery and geographics features - and those nearby I've already seen quite a few times.
I think cycling sim like that is an other perfect Rift candidate - you don't have to deal with complex control schemes, view/body movement decoupling nausea, etc.
.... 
.
