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I agree, he is implying it was a difficult march for the girls, not a walk anyone would choose to take.
I also think that, if the remains had been returned to the site after death, then that would be strong evidence of use of a vehicle.
This is also consistent with that plea from LE, for people to remember if they saw a hitch-hiker or abandoned duffle bag: Since the crime did not involve a vehicle, unlike many abductions, then possibly BG didn't have one.
However, it seems that often cases are proven because of trace forensic evidence found in a vehicle, BG may have been aware of that.