Is this ultra marathon?

Yes and No,the distances are not more than 42Km so it is not an Ultra but the challenge is doing long runs on consecutive days. That is what you have to prove to earn the title of The Ultra Dynamo

Why back to back hard runs?

To build a muscle we rely on the principle of muscle overload. It is not during the workout that the muscles are built but later when you rest/sleep the muscles build. The harder the workout the stronger the growth. When you have done several workouts in one formats you reach a platue. You have to go to the next nevel to get more benefits from the work.