Every coach I had and every book I own on running says to increase distance OR speed but not both at the same time. If "things happen" (i.e. worse than usual fatigue or injury) it's more difficult to troublshoot the cause.

We used to back off the speed a bit when we added/increased interval distance then slowly get the new amount up "to speed". Same when adding distance to distance runs... never more than 10% add and no increasing speed during initial runs of new distance.

my two cents

That said, good luck with all you do...

spazz