While I understand your difficulty with the twenty hour rule, you could possibly do strength work and stretch during rest periods between sets to improve training economy.
As far as the adaptation process, I suppose this is where the art of coaching comes in. I commend you for making sure your athletes are prepared. If, however, your athletes reach "core strength" levels at 12 weeks, why would you wait for another 24 weeks to begin with external loading?
I find almost all endurance athletes to be anteriorly dominated, and find that seeking balance by training the posterior chain/balancing force couples is more ideal as far as injury prevention and gains in track performance. Can you please elaborate on why you train the anterior side of the body? Do you find it to enhance injury resistance?