A couple things I see, one is you need to roll a bit more on to your side each stroke, but you are cutting your stroke short both in the back and in the front. In the back, you really need to extend your arm farther back in the push phase of your stroke, one of my coaches once told me that a great tricep extention push was the one thing that every great freestyler had in common with their stroke. On the front part of the stroke try entering the water at a steeper angle, and extending farther out. Try and watch these guys how far they reach both forward and back in their strokes especially at around 20 sec into the video where they show underwater, plus its a really good race from the world championships
200 Fr Worlds

Okay, here's some issues I see:

- You're right about lengthening your stroke. You need to reeeaccchhh out with that front arm, extending it all the way and then reaching for that far wall. As far as I can tell, you're not extending the front arm until it is straight, let alone reaching forward with it. You can hold it out there in a glide until your stroking arm is almost ready to enter the water. Use the catch-up and almost-catch-up drills to work on this (I'm pretty sure there's a thread here somewhere about those drills, with video).

- You're dropping your elbows before your hand enters the water. This is causing your entire arm to enter the water almost at the same time. You should enter the water with your hand first, then wrist, then elbow. If you drop your elbow like you're doing, the initial motion is pushing water down. Instead, you want to slice your hand forward into the water, extend into your glide, and let your first motion be pulling water behind you. You only want to move water behind you, not in any other direction. All else is wasted energy.

- It's hard to tell from this video, but it looks like your right arm especially is not pulling underneath your body. You don't want your hands to be pointing toward the bottom of the pool. After your nice reach and glide, your elbow should bend at about a 90 degree angle, your hand should come underneath your body and pull toward your belly button and then exiting the water by your thigh.

- You're missing the last 1/3 of your stroke, the tricep extension that pushes all the way until your thumb brushes your thigh on its way out of the water.

- Your overall body position looks good: no feet dragging, and your head is not too high. But you're not rotating at all from the hips, the stroke is really flat, just using the arms primarily. Think of rotating from the core of your body as you reach your arm forward into the glide. Rotation is what gives almost any sport its power, from Tiger Wood's golf swing to a quarterback's throw to a boxer's punch, it all comes from a rotation of the core of the body. Swimming is no different. In a standing position, reach your arm out in front of you, like you're reaching for the wall, but keep your feet on the floor pointing forwards. See how your core twists? That's the action you want. Once you start twisting your core, it will be easier to keep your elbows high on your recovery and also get a good hand entry. It all ties together with this core twisting motion.

That's a lot, all in one chunk. When I work with swimmers in real life, I usually try to have them focus on one or two changes at a time. Any more is very difficult to integrate. I would think about picking something like the core rotation/reach, along with clean hand entry (no elbow dropping) for starters. Once that gets comfortable, take another week or two to add in a longer glide and to bring that right hand underneath you while pulling. Another week or two you can work on the back half of your stroke, making sure you include the push at the end of each stroke, etc. etc.

For the lenghtening your stroke issue. One thing my coach has said that felt rather strange, but helped. Picture trying to reach over a small log/small tubular floaty out in front of you - helps to keep your hand higher, reducing the liklihood of diving it into the water too early, and make you reach forward.

For following through, I've always done the thumb to thigh drill, just dragging it across as I stroke. I'll usually do a few strokes of it in the middle of sets just to make sure I'm fully extending back...it's made a big difference.

Also, you said that this was filmed during warmup. Sometimes it just takes a bit to get things moving properly. If/when you film again, I'd definately suggest warmup and a bit of drill work, then film, then your workout.

Best of luck! :)