Plainly Lady Mary was a very, very tired horse.
She shot in to the final furlong with whip and spur lifting her on, every stroke brought a quivering response; all that was in her strong heart was going into this race.

And still the chestnut gained.

At the sixteenth her flying tail was reached by his nose And still he ate up the distance.

Yet spent as the mare was, the chestnut was much farther gone.

If there was a roll in her weary gallop, there was a stagger in his gait; still he was literally flinging himself towards the finish.