Since you just started structured training, sweet spot base 2 is actually really tough and I found to be great preparation for marathon mtb. Since you have exactly 18 weeks, I suggest doubling up on sweet spot base 2. It is not a “traditional” base phase and is really the best bang for your buck as far as generating endurance, power, and good pacing. I think you’d get more out of that 2nd turn on SSB2 than anything else. Hopefully your FTP will rise so the 2nd turn will be at a higher wattage, and thus still a progression. You can also add more outdoor volume to build up to your event. Good luck!