Breeding is something you can devote your life to if you want. I am on my fifth generation of Narragansetts and they are now 3" taller, with longer shanks and keels, and 4 pounds heavier at 7 months than the first generation, while maintaining balance, vigor, and conformation. To get there, I've had to cross in bigger Bourbon Red (resulting in Golden Narragansett) and I can get back to Narragansett in one generation.
The easy part is generating a lot of offspring for the next generation of breeders. The hard part is growing them all out until they are big enough to evaluate and ruthlessly selecting the top 10% to keep and grow out to maturity. Because I have favorites in the flock, I had to create a scorecard to objectively evaluate my stock against my goals, and then I had to sell some of my favorite adult birds because once the next generation was on the ground, they were no longer in the top 10%. I did this 3 years in a row. This is the first year I am keeping both a male and two females from the previous year because they were proven breeder quality that met the standards. I want to close my flock this year, I should have good enough stock to build from for several generations. Every importation of new stock brings its own issues that don't always present until the next generation.