In all seriousness, LoS and mount up.
A quick reflex like that can catch even the best of us off guard, and if you're using good consumables your mount should move around 130% speed, which is a helluvalot faster than I can run. Remember to hop while mounted, because if you're dismounted your momentum still carries mid-flight. I keep my mounts keybound close for situations like that, except it usually involves running away from people rather than running towards them.
And if there's no LoS objects, you ought to be asking yourself why you chose to even engage a Marksman on a clearly unfavorable battlefield.