Guitar Playing & Muscle Problems!

I'd have a look on right hand posture and guitar height first, getting tired after a long break is something normal, but then again, in few months playing you should get everything together again.

I stopped 6 years and sure, my right arm got tired when I started playing in bands again at first, but in few months dissapeared.

No magic recipe but practice and getting the right posture and guitar height according to player IMO
 
Well, I highly tend to disagree. If you're not a total pussy with no muscles at all, you won't "gain" any muscles from playing the guitar, cause there are so many other things which already provided you with all the muscles you need. Ofc you need muscles to move your hand and stuff, but this "muscle effort" is so minimal, you just need to use the right muscles and don't flex any unnecessary ones. Efficiency is the key. Many really good guitar players even claim to play with their nervous system.
All I can think is that you don't understand the difference between endurance and strength exercises. No one is asserting that you'll get popeye forearms. "Many good guitar player" don't understand the basics of human anatomy and physiology. Anyway it doesn't matter b/c the solution is still to practice whether you believe you just need to unlock the mystery of guitar playing or whether you acknowledge it as a physical activity.
 
^ lol Dude, you need to chill. All I was saying is that there is no magic guitarplaying muscle, which will make you master 250bpm downstrokes for 2h at a stretch. There are defo muscles involved, since there is no movement without using muscles, but you need to learn to play economically by using proper technique.
 
Stretch your shoulder joints and gently stretch your wrists before playing. Sit correctly with good posture so as not to stress your lower back, and warm-up for 5 - 10 minutes with simple chromatic exercise that dont take any effort to play.

Stretch again, start playing.

A common mistake is to tense up as you play faster, when actually being loose and relaxed will give you more speed, power and precision.

Dont go to the gym to get better at the guitar, play your guitar instead. You think M.J.Romeo knows what a dumbbell looks like? No, he just tied a cheeseburger to his headstock to keep him motivated.

There is no shortcut, but with time and dedication you will get there dude, if you really want to.