my ultra trail racing training includes any number of back to back long runs on weekends. the 2nd day long run is designed to be done on somewhat tired legs. for me, these long runs are done at an easy pace. back to back efforts must be trained for, and managing your effort during each is very important. eg. when I do back to backs, I think of it as a 2 day event, not 2, one day events…

Another thought … I’ve been finding nutrition and hydration to be another large component in how well I recover, specifically, carbohydrates pre- and post-event and more and better hydration than I think I need. That might be another aspect to consider. Dietitian Rebecca Dent has some articles on this site; you might find something useful there. She has written one article on hydration that I’ve found very useful. Also taking into consideration that men and women metabolise the macronutrients differently and thus have different needs (age/time of life plays a part too, but much more so for women).