If you are walking similar distances in similar terrain to what you've done in the past, and you're taking similar types of food that have worked well for you in the past, then I can't see any reason to change what works for you. As any of these factors change, you might want to consider whether the change is significant enough to warrant additional calories. If so, then getting a rough idea of how many calories you normally need would be a good starting point for figuring out the rest.