I've been bowhunting for 13 years and I've never had a circumstance where I had to hold by draw. I never draw until a shot is present. I have no problems drawing on a deer that is in the wide open. Learn to draw your bow slow and steady and that will help you much more than the ability to hold for long periods.
Then your about due. I'd feel safe in saying the time will come sooner than later!
You can hold alot longer than you'd think if you drop your drawing arm by swinging your elbow down to your side and lowering your bow arm a bit.