CO2 is heavier than air. So if you fill the keg with CO2 and keep it mostly upright, it'll pretty much stay put for the length of time we're talking about. Many people do this when racking to the keg, but I never worry about it and just purge the headspace when I have it filled. I haven't noticed any oxidation problems thus far.
What I did while waiting to get my beer in kegs was to bring one of my kegs to the local microbrewery. They filled it up for $55 and I had 5 gallons of one of my favorite stouts on tap right away. yes, instant gratification, and it was worth it!