TB and heavy cresty neck to me just don't go hand in hand...
To use a human analogy: I'm a 60+ year old guy. I run, do pull-ups and push-ups, ride horses. I'm reasonably fit. Would love to lose another 10 lbs but not going to hold my breath. If I did lose another 10 lbs, I'd be 5'8" and 140 lbs. Would NOT win a bodybuilding competition, but I'd be pretty fit.
That is my genetics and hormones at work. A few days ago, I ran 4 miles. Got home and it was 104 degrees in the shade.
Now...suppose I went to the gym 3 days a week and did bicep curls out the yin-yang. I'd get somewhat bigger biceps, but probably not much. I'm not a mesomorph. I'm "designed" to go distances. In horse terms, I'd be more Arabian than Quarter Horse.
And if I did get bigger biceps, so what? What sport could I play that depends largely on bicep size or strength?
So it makes more sense to figure out what sports I'm built for, and enjoy those sports - and let my body do what it is going to do. I've spent more than enough decades dieting to know my body tends to win out in the long run.
Like QtrBel says, a thoroughbred is bred for speed. And a gelding is going to be lower on the hormone scale for muscular necks regardless. Hills, sand, transitions, turns - they will all build up strength and agility. But they probably won't give a thoroughbred a 'Working Stallion Neck'. And I was never going to be Arnold's stunt double....