Coach Roselyn — if your workouts cannot be completed as specified, which they cannot, and you continue with workouts that cannot be completed, before long your girls will encode a mindset of impossibility. Then they will think your workouts cannot be completed no matter what you assign. Then you are screwed, because your only escape is to retreat to workouts so easy that neither you nor they will consider them to be serious. As soon as possible, get to something on the order of what dbandre suggests before you lose them mentally and emotionally. As he suggests, success in workouts breeds self-confidence and then self-confidence breeds success in both training and racing.
I would just about kill for 16 and 17 yr old girls at 25 low for 200. From there, build them, don’t break them. You can nurture a desire to run 400, but not by beating them up and certainly not by giving them workouts where failure is the inevitable result. When I give a too-hard workout, and it soon becomes clear that the kids cannot do it, I back off (in terms of pace, reps or rest) and tell them the truth: the assignment was my mistake.