You almost can't tell anything about your 5 k ability when doing something like 10 x 400. In the winter when doing only little quality it may be extremely hard running them in let's say 72 sec even though you are able to run 16 flat for 5 k! But after a lot of fast stuff over the summer your 5 k ability might just get worse even though you get able to run those 400's in 67s or even faster! I'm pretty certain that the best milers in the world will have no big trouble averaging 55s in such a workout while the best 5 and 10 k runners will find it damn tough!