the modern verb is a merger of two related Old English words, in both of which the first letters sometimes switched places. The first is intransitive rinnan, irnan "to run, flow, run together" (past tense ran, past participle runnen), cognate with (cf. Middle Dutch runnen, Old Saxon, Old High German, Gothic rinnan, German rinnen "to flow, run").
The second is Old English transitive weak verb ærnan, earnan "ride, run to, reach, gain by running" (probably a metathesis of *rennan), from Proto-Germanic *rannjanan, causative of the root *ren- "to run." This is cognate with Old Saxon renian, Old High German ...