My wife went to altitude (Kenya) from July 11 - Sept 26, 2007 - the first extended time she'd spent at altitude since leaving there in 2000.
She began training on August 4 (She had taken off since July 5) and did 7 weeks of basically base type training, mixing in one fartlek run and one track session per 10 days during the last 30 days.
She returned to the US and ran her first race on Sept. 29, and PR'd for 10k with 33:25 (20 seconds better than her previous PR).
Here's how her fitness held up over the following months:
9/29 - 10k, 33:25 (PR)
10/6 - 5k, 16:15 (Previous best on that course was 16:40)
10/13 - 21k, 1:19 (Won by 7 minutes)
10/20 - 10k, 32:56 (PR)
10/27 - Track Mile, 4:57
10/28 - AM, 5k - 17:08, PM, 5k - 16:50
11/3 - 10k, 33:42 (Won by 1 minute)
11/15 - Solo Workout, 4x1600m w/ 400 jog recovery, 5:05, 5:05, 5:04, 5:00
11/25 - 10k, 32:31 (PR - beat a loaded Intl. field)
12/1 - 10k, 34:08 (1st - 85 deg F & high humidity)
12/2 - 21k, 1:17, (1st - 11 hours after the 10k)
12/8 - 42k, 2:48:51 (PR)
1/5 - 42k, 2:48:37 (PR)
1/19 - 10k, 34:33 (2nd - tough course & high winds)
1/20 - 21k, 1:18 (1st)
2/2 - 10k, 34:21 (3rd, beat several 32:30 women, hot & brutal winds)
2/9 - 21k, 1:17 (won by 3 minutes)
2/16 - 21k, 1:20 (won by 3 minutes)
2/23 - 10k, 33:38 (1st)
3/1 - 21k, 2:37:26 (11 minute PR!)
As you can see, the benefit she gained lasted for a while. I don't recommend the racing schedule if you want to walk later in life, but the benefit from altitude was tremendous for her. Not everyone will respond like she did, but even I, Mr. Lazy, saw my resting HR get back down to my sea level norm after about 60 days at 6000+ ft. When I came back to sea level after 6 months, my HR was a good 10 beats lower than when I left.
Some of the things she thinks helped.
1. Take it easy at first - don't be in a hurry to start "training at altitude." Your body will be adapting whether you train or not, so let it have a little head start by running easy, less, or not at all for the first couple of weeks.
2. Eat well. Your body needs help adapting to the new air - give it. Might want to add a multivitamin if you're not using one, and make sure that you're eating the right stuff and plenty of it.
3. Run by time & effort, not by mileage. When you do want to run a specific distance, start your watch when you're leaving the house, then leave it behind. Stop it when you get home, don't worry with the pace during your run - let your body, not your watch dictate speed.
4. Hydrate. A lot.
5. Start short and slow with your workouts. She started w/ 600's at 5k pace, then 800's, and finally 1600's over a 30 day period.
6. When you return to sea level, maintain the running times of your workouts - don't be tempted to shorten your running times now that you're a little faster, instead run a little farther. This seems to be a major mistake people make when they do the "summer at altitude," and as a result they lose fitness before the important XC meets roll around.
Anyway, that's one person's experience, but maybe something in there will help.