I'm not sure what the USMC standards are, but US Army standard is: 12 miles in 3 hours with 45 lbs (not counting water). You won't have to do that in basic, but you should train towards that goal.
That works out to 15 min/miles or 4 mph, I would suggest you always train at that pace (give or take).
Increase weight or distance one at a time, not both simultaneously. You should try to at least be able to do 45 lbs for 5-8 miles @ 15-16 min/miles when you go to basic.
Just to give you an idea: ruck sacks in the real world usually weight 70+ lbs; my 3 day ruck sack weighs in at 80#, plus water, body armor and ammo.