OK Lewis, I feel like writing stuff so here you go. Hope things aren't lost in the translation from English to English. LOL
Here are a few things you can do to get your dog to go far and straight on the initial cast.
When you say "go out so far on a single cast" I think you mean from the initial send, correct. Technically when you send the dog on a blind it is a cast but we use the term "send" and any subsequent handles required to stay on line (the path) are what we call “casts”. When we handle and cast the dog at a distance we also expect them to carry or go on that cast for a long distance as well. Most here teach their dogs to "go as sent". That is to take "the line" (direction) I give you and keep going until I stop you and tell you different. Of course no dog is going to stay on "a line" all the way to the bird due to the "factors" that may be present. (distance, wind, terrain, cover, water, drag back scent, etc). So, much of our training involves teaching the dog to take our initial cast (send) on a precise line and to fight the factors along the way to the bird. Keep in mind our blinds can be any distance from 25 to 400 yards give or take.
One way we teach a young dog to "go as sent" is through handling drills. To start with it helps to teach your dog (at heel) to pivot with you while looking out into the field at a particular spot in the distance. Like a bush 200 yards away for example. One school of thought (not the only one) is that wherever your “dog side foot” is pointing it’s the dogs job to reposition himself so his spine is lined up with the outside edge of your shoe. This in turn tells him where his eyes are to be looking without you asking him to do so. Get it?
With dog at heel and sitting teach dog to pivot with you using "heel" to pivot left and "here" to pivot right or vice versa if dog is on your right side. Handler essentially stays in one spot and simply pivots on his feet with little other movement. Dog is to move with you using your left foot as his cue. While pivoting the dog should not get up but stay in the sit position while moving with you to any point on the compass. You should keep your physical movement to an absolute minimum and speak quietly to your dog. You must be able to look straight down on dogs head and see his eyes so the best heel position is with dogs collar at your leg and his front feet even with your front feet. Whatever it takes so you can see his eyes and know exactly where he is looking. This is sometimes different than where his nose is pointed. LOL
Hope you got that idea.
We’ll start with a wagon wheel drill to work your dog on moving with you using heel and here. Think of a "Wagon Wheel" 25 yards in diameter with 12 spokes and you and dog sitting at center of the wheel. You're going to use just 1/2 of the wheel and place several white dummies at 9 o’clock, 10, 11, 12, 1, 2 and 3 o'clock 25 yards out on closely cropped grass.
Now, with dog heeling on your left side facing 12 o'clock pivot your left foot to point at 9 o'clock. Dog should move with you and his spine should line up with and eyes should be looking at the dummy sitting at 9 o'clock. Not looking at the one at 10 o'clock but at 9 o'clock. OK? When he is looking at the right one, cue him "good" and send him to pick it up. If he flares and goes to a different one, stop him, "No", call him back and try again. Receive dog, take dummy and line him up for the next dummy, repeat. This drill helps teach the dog to move with your body cues and look precisely where you want him to look and is the start for getting your dog to go a great distance on the initial cast. Once dog has this down you can make things more difficult with another drill.
Go buy about 10 plastic traffic cones about 16" tall and paint them all white. (you can make due with something else white about the same size but the traffic cones are convenient) You're going to start this drill on short grass like a football pitch or golf course. Place cones randomly in the field at various distances and spread well enough apart so not to cause problems but have some close enough to make dog think. Distances will be from 25 to 100 yards to start with. Once dog learns the game the long one could be out 300 yards or whatever as long as he can see it. Running to these targets is going to help you teach your dog to go as sent. Be sure to use a “cue” word to let dog know he’s looking where you want. This will come in handy at the trial.
Now, place at least 2 white dummies about 1 yard in front of each white traffic cone. With dog at heel pick a cone to line the dog up on like you did in the wagon wheel and send him. This is a low pressure type of drill so if dog does something wrong, stop him, call him back and try again. Remember that this drill is also meant for you to fine tune how the dog works with you at your side at heel. If you can teach a dog at heel/sit to line his spine up with where your left foot is pointed you’ll be ahead of the game.
These two drills are fairly simple and if you want your dog to take and hold your initial cast this is one way to get there.