I've found while getting in shape after a lazy period is that accountability always helped a ton. I don't know what features besides the videos and such you have planned but one suggestion would be a daily email in the evening sent to registered users that says just one thing:

Have you done your exercises today? (or something to that effect)

Then have Yes/No links that answer directly from the email without having to login, mess with stuff, etc.

Then track those and chart them, whatever. Possibly stick in the email a short blurb.

Just tossing out ideas - I'll be checking this out when you release.