You can add volume to suit your level of fitness. Just do it by increasing hours in Z1-2. I’d suggest following a similar volume progression as laid out in the plan in terms of the percentage you add from week to week.
If you are averaging 15 hours/week and training for most of the year then you’ve likely been doing 700+ hours/ year. Have you kept a training log? Our memories for this kind of thing can be quite unreliable. If you’ve been doing 600hr/year and training 11months (about 48 weeks) then you weekly average is probably closer to 12 or so.