Leaving altitude for 20 days will this hurt me?

YOU ARE A HERB. DON'T WORRY ABOUT STUPID SHIT LIKE PSYCHOLOGICAL ADVANTAGES AND RUN FATS YOU NO TALENT PUSSY.

alright

May actually be a good idea to go to sea level for 3 weeks prior to competition if you have been at altitude for a few months. Many of the world's best say that it is good to go down and do workouts at the right speed to get ready. At altitude, it is difficult to train at marathon pace so from a confidence perspective it is not ideal (you don't really know what to expect). It is of course personal, but it may work for you.

A common strategy is to leave altitude about 3 weeks before a big race and during the taper to run high intensity runs at sea level (SL) to regain some leg speed. For the marathon, this is probably less of an issue than for middle distance races.

Suslov (a Russian researcher) looked at altitude training and SL performance and found some interesting trends in the athletes (Soviets) that went to altitude and then came down to race. He wrote the findings up in an article for New Studies in Athletics (I would imagine NAU has the publication or could get a copy of the article--a Sport Discus search at the library would give the pertinent information). He found that there were periods of time following a return to SL that performance was good and bad and that the effects lasted out to 35 days (I think). This is consistent with the literature I have reviewed on the topic.

don't think so much

3 weeks down from altitude is perfect to race. I've found that the 'window' is 12-21 days. You'll fly round the Half.

20 days at SL, post-ALT is just fine for racing. Don't worry.