I'd highly recommend getting the book "Altitude Training and Athletic Performance" by head USOC Physiologist, Dr. Randall Wilber. Everything you need to know is in that book, if you want to get the most out of high altitude. To address your questions, you need to go to at least 6900 ft.. Anything below this altitude provides mixed results (you may or may not benefit). You also need to stay a minimal of 4 weeks at high altitude. According to the research, the benefits last ~3 weeks, but considering the lifespan of RBCs (120 days, but I've heard 90 days for elite distance runners) the benefits could last 3-4 months. Any event that is mostly aerobic, including 2 miles, is going to get a boost. However, you need to make sure you maintain basic leg speed (~strides, hills, short intervals, the longer threshold stuff is much more difficult at high altitude). Start an iron supplement + Vit. C (OJ) a month before you go up. You end up with a greater total blood volume (more RBCs) because of the EPO surge at high altitude.