Well No subject of Equestria want their Alicorn Princess be not healthy, So Training them is important job, Alicorn have Unicorn ,Pegasus and Earth pony magic in them, so to master it, she need to train in all 3 but Twilight only master her unicorn magic side but she still need to master the two other side of her magic!
Twilight had to get in shape for the Running of the Leaves, and Rainbow Dash decided to train her, but as she started dozing off with Twilight’s lack of athletic ability, she gave Trixie her coach outfit and had her train Twilight.