Disclaimer - I'm not a DK player
. Doublecheck everything I say.
I did a quick check and no DK-specific legendaries seem to affect runspeed. So you will be using
https://www.wowhead.com/item=132443/aggramars-stride (as any plate speed twink would), and then you have the choice of
https://www.wowhead.com/item=146667/rethus-incessant-courage or
https://www.wowhead.com/item=132452/sephuzs-secret . Rethu's has 15% and 2 more sockets, but loses its buff around adds. Sephuz is a fixed 10% with an additional 70% sprint for 10s every 30s which you can trigger via stuns.
I'm not quite sure how DK secondary stats scale (so as to pick the best secondary stat to stack), so someone else will have to chime in here. In either case, 111s get the full 50 stats from BFA gems, so a 111 should end up with a slightly higher runspeed, all else being equal.
If you don't have the 1hander Garrosh heirlooms (so you can dualwield them for 4 extra sockets), then go UH or Blood and use
https://www.wowhead.com/item=13505/runeblade-of-baron-rivendare . If you do have the heirlooms, check if the extra 4 sockets will outweigh the flat 10% from the runeblade. Pick any cloak that can be enchanted with the 2% runspeed Gift of xxx enchant.
Those are the major points, I think.