Speed racking takes a lot of time and patience to do it properly. Start out doing a lot of walking interspaced with some slow singlefooting. I'd spend at least 2-3 weeks just slowly building up the muscles and endurance. Then spend another 2 weeks asking for small increments of speed then reward her by going back down to the slower rack, don't forget a few mins of walking to let her back relax.
It takes up to 90 days to get a speed racking horse and keep them sound so do not skip on conditioning.