@shotimac
As with all new things in life we tend to go in head first, overwhelmed by our new found hobby and willing to do it all to get to a descent level we injure and burn out.
The best way to learn something new is to ease into it. An arm not conditioned to play music now has to work for around 3-6 hours and you expect it to be fine? Sure some of us could do it but the majority wont. Fatigue is a good indicator, you overloaded the system now its time to recover and prevent getting an injury which takes longer to heal from.
At minimum cut your practice time in half, split your daily session and dont do them in one sitting and than slowly build up again. Make sure you get a solid stretch on the forearms and working out can do wonders by keeping the body balanced.