How often should I do cardio for conditioning and endurance and how long for?
Cardio should be a staple in your routine.
If you want to be a long distance runner/cycler/tri-athlete etc, then you should aim to do cardio at least 3-4 times a week, and most probably every day.
This should be long, sedate cardio. It doesn't need to be done as fast as possible like sprinting, rather it should be at a pace which you can maintain consistently for between 20-45 minutes or longer.
Be aware that if you want to do cardio AND GAIN MUSCLE, you should not be running as much or for as long.
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