How is best as a participant to prepare for the retreats?
Recommended but not required to prepare your body would be to try and eat healthily and do some exercises to keep your body flexible (nothing specific just whatever exercise you enjoy most). Mentally you can prepare by finding some time each day for inner reflection and to be aware of how you are feeling. Preparing your body and mind will help you to get maximum benefit for the retreats, but it is not a requirement, because after these 3 days you will find it much easier to continue with a healthy lifestyle.