Can Guinea Pigs be left alone to run around the garden?

I might be getting 2 guinea pigs soon, and I was wondering if instead of getting a run, you could just let them run around the garden. They will still be able to go into their hutch if they want to and I won't always be around to check on them. Is it okay? Thanks in advance! Plus, there is a cat that sometimes walks across our back wall, would they run away?