I would have to agree with the start slow strategy! It is really easy to over work the knee flexors when running in the water. I would start with 10 min. and work up in 5 min. increments or less. Using a flotation belt will make it a lot easier. I prefer not to use one but you may want to alternate using it and not using it until it feels easier without it. You will work your cardio more without it. Hope this helps.