The process of building muscle (either strength or size) requires disrupting homeostasis. Since you have put your body under a certain amount of stress to achieve this level of adaptation, changing that stress will cause your body to make other adaptations. For example, the 16+ rep range has to do with anaerobic endurance more than sarcoplasmic hypertrophy (8-12 rep range) which you used to achieve this level of homeostasis. That said, the adaptations you are doing may be beneficial in the long run.
The take away is that you are trying to maintain homeostasis. That means the current levels of stress should not fundamentally change.
As to the role of protein, understand that your muscles are living tissue. As living tissue, old muscle wears out and is replaced by new muscle. You still need protein to maintain, but perhaps not as much as you needed to build. The number I was told was that for maintenance (of sedentary people) you need 1g protein per kg of lean mass (or roughly .5g protein per pound of lean mass). If you have 190 lbs of lean mass that would be 85g of protein. What that would be for non-sedentary people I'm not sure. Perhaps start by splitting the difference.
Beyond a certain age (around 30-35), you will deal with sarcopenia, or the deterioration of muscle mass. For people who lift weights, that should be about 5% decrease per decade as opposed to 15% decrease per decade. What this means is that you will need to compensate your work by at least 5% per decade (and increase the protein to help with the adaptations) to continue to maintain.
This is of course theoretical based on Seyle's theory of adaptation syndrome (the core theory that all other exercise theories are based on).
Running/Sprinting - Sprinting is an anaerobic exercise like weightlifting, as such it won't cause the body to change the metabolic pathways it built up. However distance running is an aerobic activity, so it will change the way your body is getting its energy. Running in general has a very limited range of motion compared to squats, lunges, leg presses, etc. What this means is that only the muscle used in running will be maintained, and the muscle not used will begin to atrophy.
Muscle Memory - Your muscles will remember how to get strong if you completely lapse for an extended period of time. What that means is that you will get back to your current peak shape faster than the first time you worked to get there. You'll have to push yourself eventually, so this works to your advantage.
