Jeffrey Armbruster wrote:
But, does no one here feel fatigue after several hours of practice, and some stiffness if you pick up the guitar early on the next day?
I'm sure you're not talking about the general morning stiffness that goes away with warmup, as ice wouldn't affect that. But that's a lot of practice! Anyone would be fatigued if those several hours were consecutive. This is enough time that you'd need to manage your physical and mental fatigue. "No pain, no gain", OK, but listen to what the pain is telling you.
If you practice while sitting, like most of us, then it's good to get up every twenty minutes or so--stretch, walk a few steps, stretch, flex and rub the hands. If possible, combine with breathing, voice, and face relaxation. All that takes less than 30 seconds. After an hour, the break should be at least a few minutes.
I've read that after two hours, the benefits of additional practice begin to diminish, and musicians benefit from a substantial break between hours two and three, meaning putting the instrument down and doing something completely different for at least an hour, ideally including sleep. This allows mind and body to recover, and the brain to assimilate new learning.