Brisk walking IS good cardio. As long as it feels like a workout. Dankar walked for a majority of his fat loss, and he did it first thing in the AM.

I usually tell people to go for a minimum of 20 minutes, but do what is comfortable for you. Just be sure to increase it a little each week so that your body won't adapt to your level of exercise.

That's a basic answer ... I could get more into it, but that should do you for a while.

As for the ephedra, check out the THERMOGENICS thread at the top of the EXERCISE forum.