Our council has the rule 1000' of gain = 1 mile, 2000' of loss = 1 mile. Some of my trips are 6 days, and only 40 miles or so total, but I call them 50 milers and no one checks the math. Some of our leaders do 50 milers in 3 days. A 6 day trip is different from a 3 day trip. Younger boys can to a 6 day 50 milers, plus for old geezers, 50 miles in 6 days is easier. 50 miles in 3 days is a marathon, not a fun vacation.
What we did that was different from most troops was get 11 year olds on a backpack early on, and have backpacks as our monthly campouts. The troop previously had car camps as the monthly trips, and the few backpacks were really just a guy and his family going backpacking. They were not announced early, so nobody could plan on joining them, and who wants to do 50 miles in 3 days anyway? We found that 11 years olds could do backpacks of 2 miles with no problem, and after that they were ready to do backpacks of 4 miles the next time.
I fear that our 6 years as a backpacking troop may be slipping away.
As far as mileage for merit badges, that is strictly miles hiked, no adjustment for elevation. For counting as a 50 mile hike, I use the elevation fudge factor, and the mileage inspector from BSA has not audited us yet.