So, I can do gallops prety descently for quite a while without getting to tired, but I've been doing exercises from andy james's dvd metal rhythm guitar in 6 weeks, and there are some exercises that last 1:40 minutes, and I am supposed to do gallops and downpicking combinations for all that time. My hand gets tired, because I'm not that good at downpicking. So in order to build stamina, is it supposed to "hurt" a bit. I mean it doesn't REALY hurt, just get tired, if you know what I mean
Downpicking is very unorthodox for quick picking like that, and for some weird reason everyone does it -.-

Anyway, it all comes down to stamina, really focus on your arm too. If you're relaxed at the start and slowly start tensing and tensing it'll hurt. It's like running, when you start to feel tired you try harder and harder so your legs are at their limit. Same goes for your picking hand. Train it to get more endurance, practice-over-time