Ya, as suspected, these were not easy. I'm not sure how you did it with the 120's, I'm even more impressed after giving them a try. I did my 60lb DB (homemade concrete circus DB looking thing with a 2" handle) for ~60yards with each arm and that was PLENTY. I did 3 rounds of 60 yards each hand, then a 85lb sandbag carry for the same 60 yards (30 each way - 1 turn).
But it was a fun one, so I'll likely start adding these in once in awhile as I feel like even with the light weight I'll be getting some good shoulder benefits from these.