This is a common problem for more experienced athletes with more time to train. I personally think the current “high volume” plans are not high volume by any standards. You are unlikely to find a coach that will call 8-10 hours of training each week “high.” I made a post about this a few weeks ago
and there is a prevailing opinion that not enough people need a plan like that for TR to explore. I am not sure that I agree, but I don’t have the data.
I have to nearly double the volume that is provided to me in TR to maintain my CTL. I generally add endurance miles which should not tax the system too intensely and I will strategically select longer or stretch/breakthrough interval workout variants to take on a little more load. To do this you need to be pretty diligent about listening to your body and monitoring your ATL, things can get out of hand. I find that if I am disciplined to making one day of total rest from training sacrosanct (typically Mondays), I am able to maintain a good ramp rate and shed the excess fatigue before burnout.
The responses focused a lot on the plan providing the “minimum effective dose” of training to work relevant energy systems which I understand thoroughly, but at the end of the day, a reasonably structured 15-25 hour per week athlete cannot yield the same results reducing the volume to 10 hours a week no matter how well periodized the plan.
Short answer, as @BCrossen stated, you’re going to have to supplement while focusing on maintaining the quality of those working sets.