The Abarbanel in Bereishis understands the aforementioned Rambam to mean that the six days were not 24 hours, but rather six steps of creation. I believe there's a Professor Schroeder IINM who quotes that as a Torah source for an Old World.
Have you read the Abarvanel?
First of all he understands the Rambam to say that the world was created complete on the first day (a.k.a. last thursdayism) and then the next six days the six levels of creations were "celebrated" (Light and Dark, rain, minerals, plants, animals, man).
Second of all he blasts that interpretation completely saying
ואומר שדעת הרב חוץ ממעלת תורתו הוא שקר מבואר, ויתבאר זה מפנים: הא' שהכתובים בספור הבריאה מעידים ששה פעמים ויהי ערב ויהי בוקר יום כך, ואיך יכחיש אותם שום בעל תורה לומר שלא היה שם מספר ימים ולא ערב ולא בוקר? האין זה הכחשת הכתובים והכזבתם?
and then brings 5 more refutations to his understanding of the Rambam (including, interestingly, the question I brought all the way in the beginning about keeping Shabbos on the 7th day).
And if someone will tell me that "ok it's a Machlokes between the Rambam and Abarvenal and I follow the Rambam", most Meforshim don't understand the Rambam that way so it's only a Machlokes between the Abarvenal and the Abarbenal's understanding of the Rambam which he discredited in pretty strong terms...