you can do SSB1 LV, then double up with Z2 work. 12h splits, means 2 a day. So 2 workouts every day of the week. 1 SSB1, the other something like Pettit or Colosseum -3. On the off days in SSB1, double up the Z2 work.
Saturday, opt for the 2hr version instead of the 90 min, in the plan, and do 1 a day. Same for Sunday, add a 1-2 hr workout, or do 2 1hr rides. Z2. The longer rides on the weekend is preferable.
Gauge every day. If you feel tired. take the day off, but try to stay on for the SS work and take the days off where you have the Z2 work.
What you’re doing here is bumping up CTL, volume, getting the aerobic side to produce more.
I prefer to stack more Z2 than intensity until you can get that CTL up, then you can start to drop weekly TSS but increasing intensity. I don’t know what your weekly TSS looks like, but doing the Z2 fillers help a good bit.
You can also shift the plan days. Let’s say, T/Th/S are intensity, but you have something come up on Th or you feel tired, so you can move it to Fr, or work in a Z2 instead, if you need. Then on the next week, you may know it’s a bit much, so you can cut back the 60 to 45 min Z2 work, and try to keep on track for T/Th/S. Your body isn’t going to hate you so much if you move days, and some days are faster recovery, and some days are harder, all depending on what else is happening around you.