2005 Subaru Forester P0420 Code

My wife has an 05 Forester with 62k miles on it. Recently the Check Engine Light came on. I checked the gas cap, filled her up with gas,and went for a drive and the light was off. An hour later she took it for errands and it came back on. Last night I brought it to a friend and he plugged in his computer and got the Po420 Catalyst Efficiency below threshold code. The only thing I have noticed with the car is it idles a bit lower than it did before the light came on. He reset it and I took a drive of about 12 miles and it did not come back on. I checked to see if the Catalytic Converter was glowing. No. I checked to see if the exhast was abnormally hot. No. The light has not yet come on today but it still idles low. My concern is spending the money (that I don't have a t the moment) right before the holidays. My wife is driving to VT with the kids Friday, 8 hours, for X-mas with me to follow on Saturday becasue of work. Is this something that can wait a week or two before getting it fixed? Should we be concerned with the long drive? I assume that if it was serious the light would have come back on after the reset. When I drove it after the reset I drove down the highway and got it up to like 90 for a few miles and no preoblems. Any ideas? I have no mechanical ability, pathetic because I live in maine and most mainers a pretty self sufficient. Any help would be much appreciated. Thank you, Brad