grashooper hits the nail on the head. Therein lies some of the issues that people with low training loads don't get. That being runners who take the sport on a more dedicated level are always many steps ahead (also why people who get injured lots have problems) and that is once u get to a certain level of aerobic background u have a lot more to take out. Of course there always has to be some buildup, but those who have a large training background can get away with not doing the buildup as long or as 'traditional' egs you can do some more intensive work than if coming off a low training load or a long term injury….
….but to answer your question approx 8-12 weeks for most…..