Always aim to do cardio AFTER your workout.

This is especially important for MUSCLE GAIN, as doing cardio before you lift can hinder your ability to shift your weights around.

Doing it after your workout will actually be a good thing, because your muscles will be warm and ready to move about. You can also do cardio as a separate training session.

For endurance, it may be a good idea to implement HIIT cardio into your CIRCUIT training too, to work the muscle while it is even more fatigued.

Related Questions:

1. Can I do cardio and gain muscle?
2. What is the best cardio for losing fat?
3. Should I do total body training for conditioning and endurance?