I think that if you're doing a marathon in early fall then working out at altitude is great. I catered my previous post to a collegiate cross country runner.
If you're a sea level guy going up in the mountains to train for a marathon three months away then you just need to make sure that you can do enough race pace work for the marathon distance, i.e. tempos, repeats. If you're the type of person who is very familiar with their own lactate threshold then you will need to adjust your expectations for splits once you reach altitude.
For instance, if my lactate threshold pace is currently around 5:20/mile for a 10 mile tempo then I will need to lower that pace significantly at altitude, to maybe around 5:35. The simple reason is; less oxygen, means the body is working harder, means that you'll get anaerobic quicker lactic acid will build up much more quickly than at sea level.
You can return to sea level and race immediately if you've done the work and aren't broken down from altitude training. There isn't a specific timeframe that you need to spend back at sea level to "get ready". But, if you come back to sea level and have 2 months before you run a marathon then you may lose some of that extra oxygen carrying capacity. That doesn't mean you're losing fitness. Simply losing those extra red blood cells you body made at altitude. I would read up on when the best time to come down from altitude for a marathon might be. I'm not sure.
One thing I will say about returning to sea level; If you train at altitude in cool dry air and then return to a humid place like the midwest, south, or eastern seaboard, then you will still feel like you're running through soup. I've heard of pros coming into big races in the summer a week before hand just to get used to the humidity.
There isn't really a need to get super scientific about altitude training. It works. But you need to monitor bio feedback much more closely up there. You can't run through fatigue as easily and you'll break down quicker and for longer. Recover slowly (7:00 + pace), hydrate a ton, sleep more, don't do anyone else's training, and if you start to feel an injury coming on take a couple days off. Better two days off now than two months later.