Why not follow the MTB marathon plan and instead of doing the 2.5h indoor ride, go outside and ride at endurance on a MTB trail and work on skill (if you are not doing so already)?
Doing that 2.5h indoor ride with varying intervals, i assume you do that to simulate a MTB XCM, which i guess is fine to do sometimes as a simulation, to figure out fuelling, drinking, etc. But would not make that a regular thing.