Help? What should I do?

So, I'm dog sitting two huskies. One is 50 lbs and one is 107. They are completely untrained and very stubborn. But, I decided it would be ok and it was for a lot of money. The walks are hell and I dread them. The 50 lb one constantly pulls to the point my hand goes purple because I'm holding the leash so tight. He often bucks to take off running and I have to pull him back or grab him while walking. I thought I was ok and got the hang of it. But, today we saw another dog walking (the owner told me they are NOT to be allowed around other dogs; the smaller one has a history of incidents) and I was holding them while the walker nonchalantly went about his business. But as I was walking towards our destination and being constantly pulled by both of them now, I fell into mud and might have hurt my ankle and now I am feeling defeated and at a loss on what to do. I am overwhelmed completely, but I do not want to quit this job, should I contact the owner and say I cannot walk them anymore, or walk them one at a time? I just want experienced opinions since I am fairly new to Rover (mostly dog sat friends and family pets I knew). Thanks.