If you don't yet have a solid down command you don't want to be increasing difficulty by adding distance or duration. Work on getting a consistent fast down first, and then start making it more challenging.
Personally, I prefer not to physically manipulate the dog, I wait for the dog to offer behaviors and then mark and reward ("capturing"), and also lure into position. With capturing, once the dog is starting to offer downs more frequently, I start to put it on cue by naming it. For example, she lays down on the floor, you say "yes!", and toss a treat a few feet away so she has to get up to get it. Then wait for her to lay down again, rinse/repeat.
Once she figures out the game she should be slamming herself into a down in anticipation of the reward, and at that point you would say "down" right before she does it.
With a lure, I like to get the food out of that hand as quickly as possible so the dog doesn't become dependent on the sight of food in my hand in order to comply. I'll lure a few times, and when the dog is consistently following that hand to the floor (mark and release the food when butt and elbows touch), I'll use the lure motion with an empty hand and then reward from the other hand. I gradually fade the lure motion, and it becomes the hand signal for that behavior. Again, once she's consistently responding to the hand signal, add the verbal cue a second or two beforehand.