。
She got on her bicycle and rode off.
Literal
She [topic-は] bicycle [on-に] rides [upon doing-と], ran-off-completely.
Two grammar points worth pausing on. ~と after a non-past verb (here 乗る) marks 'upon doing X, Y immediately follows' — narrative consequence, not 'whenever' or 'if.' This と is one of Japanese's most useful narrative devices, threading sequential actions tightly. 走り去る ('run away, run off') is a compound verb of 走る ('run') + 去る ('leave, go away'); the ~去る suffix productively forms 'leave by V-ing': 飛び去る ('fly away'), 立ち去る ('walk off, depart').