I used to think that I just needed to build more endurance to play fast for long periods of time in my right arm, but after playing in a pretty fast death metal band for some time I think my technique is just shit.
We played our first show a few months back and my right forearm was just tense as could be and my right thumb muscle basically was too strained to even hold the pick just into our second song. While I know part of it was being nervous and tensing up on stage, I have also noticed my endurance varies quite a bit even in practice.
I learned guitar/bass mostly by myself (and some lessons with Bob Balch of Fu Manchu when I was a teenager!) so I am thinking I picked up some bad habits along the way regarding picking and just picking arm fundamentals.

You should move your arm as little as possible and focus on your wrist. You should also hold your pick at an angle when you tremolo pick to reduce resistance. I don't know if that helps but that What I was taught.

I had a similar problem when I first started gigging. What worked for me was to take all my riffs and slow them down with a metronome, painfully slow, and to build up from there. If you have the technique down slow with no pain, you just build from there up. It sucks and takes time, but what doesn't? haha

Try a longer strap length, especially if you have long arms. At first your picking will sound mushy and inaccurate, but if you practice that way for a while, you'll be able to play fast without straining your arm as much. I find that the ideal strap length for extreme metal is pretty low.

Do you work out? It's pretty essential if you want to play physically demanding music in the long run. You will inevitably sustain injuries, and being stronger and having more muscle will definitely help to mitigate that.

I love Hand and Arm Exhaustion, their split with Cock and Ball Torture is classic.

I actually just started heading back to the gym so I am getting in better shape and hopefully that will help a bit.
No one else has presented the strap length idea and I think you might be on to something there seeing as im 6'4". Gonna give that a try! Thanks a lot man.

Well done Droneriot

Are you stretching out your hands, wrists and arms before playing? It's like any other physical effort, you need to stretch out & warm up before you start or you'll for sure have problems.

^I do very little of that actually, to warm-up I basically start playing but at manageable speeds and intensity and slowly increase it. It also helps mentally for you to do so since you're getting in the head-space of playing.

Typically, I warm up like so: Burning Times-->Violate-->Coma of Souls. And just improvise some solo stuff to get my fingers stretched.

I disagree about stretching. I'm under the impression that it's definitely not necessary in order to avoid injury, and that it's in fact detrimental to performance at least as far as strength is concerned.

I don't think it's beneficial to stretch before playing, but there may be some value in doing it at other times.