I have been going to battle
I have been going to battle with my 08 Prius with 438,000 miles on in and the 0420 and 0421. I am fortunately able to tell you that running with the codes can, in the long run, mess your car up. I noticed that the oil was being consumed at a higher than average rate. Even with the miles on it. I bought a cheap aftermarket Cat Conv and put it on and it was not as long as the oem one so I had to make a spacer. One thing that I noticed was the power was very lacking due to the A/F ratio being off due to the code. So yes you can drive it but no, it will not be good on the car in the long run.