It depends what you’re looking for on a retreat. Each adventure retreat will detail the range of activities, or the kind of distances you’ll be covering on a daily basis. You can find retreats that cover more leisurely hikes but generally the mileage will be for those well-versed in covering longer routes.
Do some research and be prepared before you book! Of course, no one will force you to cover distances you don’t feel up for, but if you’re not in favor of completing a fair mileage, then a different kind of retreat might be more suited to you. Also consider the terrain you’ll be trekking – if you’re used to walking long distances on flat paths then rough routes high in the mountains may not be for you. Or they could be the making of you!
It’s not all about trekking of course – depending on your chosen destination, you can look to experience a great number of outdoor pursuits.