Get a horse to 3*Long
Borrow a parent with 9*Long and breed another 3*Long (only 3 attempts a day)
Now you have your own 9*Long horse
Borrow 9*Long again and breed it with your 9*Long horse (for max affinity and keep of blue/white sparks) to produce another 9*Long Horse with the whites and blues you also got from your borrowed horse.
Use your 2 9*Long horses with all the blues (hopefully stamina) and whites to breed your runner.
Don't get long S because you got silver inspirations twice.