- no significant weight loss over time
- no chronic hygiene-related problems
- no chronic physical ailments related to poor nutrition or hiking practices
- no chronic discomfort (physical, mental, emotional)
Potentially chronic conditions need to be recognized early on and treated early. These are things like severe blistering, inflammation of joints and ligaments, malnutrition, chronic pain, etc. As soon as you discover a recurring condition of any kind, pay attention to it and see what you can do about it while it's still in the beginning stages.
Needs arise all the time while backpacking. Most are physical -- the needs to eat, drink, defecate, urinate, cool off, clean up, get that thorn out of your sock, etc. A rule that I formulated while doing extensive solo hikes in Ukraine was this: as soon as two needs are activated, do something about it. Before that, I would frequently find myself in situations where I was hungry, thirsty, hot, dirty, and needed to defecate -- all at once. This can be overwhelming and leads to a loss of morale and coping ability. When only one need is activated, satisfaction of the need can be put off for a little while, but two leads to a worsening of the mood (at least for me). Staying happy while backpacking seems to boil down to basically satisfying one's needs on schedule.