Instead of working her on a line why not use a round pen or even an arena and free longe her? Its easier on her joints, you can trainer her to move with your body language and position and it really only takes a few minute a day especially if the filly has lots of turn out ( which she should anyway, after all she is just a baby)

I don't think a filly would need 15 mins of work on a longe line. Youngesters don't have a long attention span so if you can work her a few minutes, get something done, then walk away.  Personally, I don't like longe line classes but thats just me.

Make sure she really knows how to lead before starting the long line training...including reading subtle body language and verbal cues.  (This will save you time and her joints from long fights over these topics.)

Free work in a round pen is good but will not fully prepare a long line baby.  Some good rules of thumb are that you needn't feed them "rocket fuel", feed that is over 16% protein can cause babies to get hot.  Always watch her pastern/cannon bone joint, any swelling in the shape of a donut around the ends of the cannon bone should be dealt with ASAP, the condition is described below and can indicate feed problems and overwork.  Do your research and monitor your baby's little legs daily!

I also believe that a larger percent of futurity babies suffer from premature lameness due to overwork of their immature bodies.

Epiphysitis
The growth plates located at each end of the long bones are also called epiphyseal plates. An inflammation of these growth plates is called epiphysitis and is characterized by swelling near the ends of the bones. Although the youngster may be lame during the active phase of epiphysitis, the lameness diminishes once the area ossifies. The area will often be permanently enlarged leaving the youngster with knobby knees or ankles. Epiphysitis usually affects weanlings and yearlings, however it may affect horses as old as 2-1/2 years of age.

Your veterinarian will probably suggest some changes to your youngster's diet and will probably prescribe stall rest for four to six weeks. She may also suggest changes in the way the youngster's feet are trimmed. The dietary changes often include a reduction in protein and reduction or elimination of grain and concentrates.