According to this article below, a weighted vest can improve short and medium distance running speeds, but I want to get in shape to carry a 45-55 lb pack at high altitude for 7 hours per day. I have three months to prepare.
… The initial results were negative. After four weeks: the runners needed more oxygen to run at a given pace (that is, their running economy had deteriorated). Similarly, their leg muscles were producing more lactic acid, a possible sign of muscle fatigue.
Fortunately, Rusko didn’t give up at this point. For the next two weeks, he asked his subjects to take off their vests and continue training as usual. A retest at the end of this two-week period produced far different results.
This time, lactate threshold was 2 percent higher (meaning that the runners produced less lactic acid), and max V02 had also increased by 2 percent. Two other important measures also improved: Endurance while sprinting soared by 25 percent, and stair running speed, a good indicator of leg-muscle power, increased by 3 percent.
… Wearing a weighted vest has the potential to strengthen your leg muscles, make you faster, improve your kick and make you a better hill runner. However, it’s important to bear in mind the following points:
* If you decide to try using a vest, wear it as much as possible during your daily routine. Wearing a vest only for workouts may not provide enough stimulation of fast-twitch muscle fibers.
* Wear the weighted vest during some, but not all, of your training sessions. The vest actually slows your average speed during training, so constant use would eventually teach your muscles to work at a slower rate.
* You’ll have to stop wearing the vest for several weeks before you see positive effects. Your body needs a “furlough” to recover from the extra stresses of vest wearing.
* Don’t try to substitute hand weights for a vest. Hand weights may slightly increase your oxygen consumption, but they won’t do anything for your leg muscles or increase your max V02 to any significant degree.
* Don’t attach weights to your ankles. The extra poundage could cause injuries and may wreck your running economy.
* Weighted vests will probably prove most beneficial to middle and short distance runners. Distance runners may be able to improve their lactate threshold, max V02 and kicking ability by using a vest, but they must guard against loss of running economy. You can buy weighted vests in some sporting goods stores.