跑马 is a separable verb (离合词) formed from 跑 'to run' and 马 'horse'. Its core meaning is 'to ride a horse' or 'to gallop on horseback'. In a separate, euphemistic sense it can mean 'to have a nocturnal emission' (dated/medical). It also appears as a noun compound referring to 'horse racing', but that usage is distinct from the verb-object structure. As a separable verb, you can insert elements between 跑 and 马, e.g. 跑了一次马 ('rode a horse once') or 跑了三个小时的马 ('rode a horse for three hours').