A run length is the straight-line distance a cell covers at terminal speed before reorienting, so it is speed times persistence time. With speed 4 m/s persisting for 3 s, the run length is 4 times 3 = 12 m exactly for the stated toy motion ledger; real active matter has fluctuating propulsion, changing orientation, boundary effects, fluid environment variation, and sampling noise.
A persistence time converts speed into one deterministic run length. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.