Historical records recovered by the Jaffa from the System Lord Baal's archives does not mention the existence of Lanteans. As you know, this contradicts the records found on Atlantis, which suggests to me, and other Tau'ri historians, that the Ancients had abandoned their identity by that point, and fully blended into the general population.

However, Kirsh'myas from the Central Tok'ra University has recently launched a controversial theory, stating that at least some Lantaens maintained their heritage, and consciously chose to ignore the kidnapping and enslavement of the population in order to focus on Ascension.

As for the Asgard, we know exactly what happened, thanks to the Asgard core recovered by SG-1: They would apparently hold position at the edge of the Solar System, and mingle with the locals when the System Lords weren't present. Due to the Goa'uld symbiotes' aversion to cold, the areas frequented by the Asgard were largely ignored.

Basic geography of that time tells me that Asgard was another realm in this era, and Midgard was on earth.

Source: am Heimdallr.