It depends on how challenging the trek is. In general, you should be comfortable hiking for about 5 hours. To see the level of difficulty for a given trek, you should review Detour’s Difficulty Rating, which is on each trip’s overview page. Also, customer’s reviews can be helpful.
To prepare, a regular exercise routine that includes cardiovascular training will be extremely helpful. By starting this three months in advance of your departure date, you should find yourself much more comfortable tackling a multiday trek.
