So, the client will be walking inside their own home, i.e. they have a treadmill?
Maybe have your client get a Fitbit or heart rate monitor and teach them how to work out at their desired intensity (give them a circuit, a steady state, whatever homework you have set for them), and then let them play their own music.
I wonder if you could make something like this during a session. So the client would get the feel of the intensity and workout that you want them to do, and you audio-record it with her ipod or other technology (phone). Then, she/he could play it back and remember exactly how fast to go, how it felt, etc. Good neural programming, especially if client is a creature of habit.