What is the longest distance you recommend running, for what amount of time, and what apparel to wear?

You should be able to run 6 mi in 44:06 just wearing PT gear, ruck 6mi with ABUs a weapon helmet LBE and 55 lbs ruck at a 15 minutes a mile pace. For training I would run up to 10 mi at 8 minutes a mile pace at a minimum and for the ruck just keep it to the standard.