
HalfOrcHeavyMetal |

Larceny. Steal a fast horse as well as the armor, so you can get the hell out of dodge quick.
But in regards to the Op, as has been mentioned above.
Mithril reduces the movement penalty speed, amongst other things, of the METAL armor it is crafted out of. Since you have asked how this interacts with the Fighter's Armor Training Bonus, the Fighter luckily has the prerequisite feat to wear the armor, and at 3rd level, Armor Training kicks in and allows, amongst other things, the Fighter to move at his normal speed in Medium Armor.
Mithril (reduces weight category in regards to movement speed) + Armor Training (allows the Fighter to move at full speed in certain weight-classes of armor) = Fighter able to move at full speed in Heavy Armor well before 7th level.
I hope that helps.