Khorne Berserkers dont have the Fleet special rule; according to the codex, they have Fearless, Furious Charge and Mark of Khorne.
Even if they did have it, you arent using it properly. Fleet of Foot is not another base 6" of movement between the Shooting and Assault phase. Per p.75 of the
BRB:
"There are many variants of this rule: Fleet of Foot, Fleet of Claw, and even Fleet of Hoof. Title aside, all models with these abilities are treated the same. A unit with this rule may assault in the same turn in which it has run."
Since Khorne Berserkers do not have the Fleet special rule, the most you could have moved was 6" (movement phase) +
d6 (Run in lieu of Shooting). Units that do not have Fleet can not assault following a Run move in the Shooting phase, while units that DO have Fleet (like many/most Eldar) can Move 6" - Run
d6" (instead of Shooting) - Assault.
That said, your misunderstanding of the rule took 12-18" of precious tablespace from the Tau force, which may have had a significant impact on the outcome of the match.