[ Etymonline :] To take (something) in stride (1832), i.e. "without change of gait," originally is of horses leaping hedges in the hunting-field; figurative sense attested from 1902.
TAKE STH IN YOUR STRIDE | meaning in the Cambridge English Dictionary
to deal with a problem or difficulty calmly and not to allow it to influence what you are doing:
I know nothing of equestrianism. Horses obviously stride, but why was 'take in' adopted to signify 'horses leaping hedges in the hunting-field' and then 'without change of gait'?