Snubbie and noKnees pretty much nailed it. Training with a device that restricts your breathing does you no good, and it actually keeps you from working out as hard as you can. Your body acclimates to altitude by producing more red blood cells to carry oxygen, and it takes a few weeks to fully acclimate. But you can feel a difference after a few days, I know I can feel a difference every year on about day three or four.
Think of it this way--even if a snorkel does mimic altitude, if you worked out with an oxygen-restricting device every day for an hour, your body would acclimate to the conditions you spend 95% of your time in (sea level) not 5%. The ideal for endurance athletes is to train low and sleep high, so that they can train hard with more oxygen, and partially acclimate by spending time at a higher altitude.
So the best thing you can do is get to your hunting spot as many days before the hunt as possible. I haven't seen any info. on this, but I also believe it can't hurt to eat foods rich in iron since your body needs it to build red cells. Best excuse there is for a juicy steak every evening for dinner.
Your training will certainly help, you'll recover faster and be able to hit the hills day after day without cratering.
