Sheldon Brown reckons that the average Walmart bike covers less than 20 miles in it's lifespan. Or rather that "most" Walmart bikes do that. It wouldn't take many high milers to skew the mean a lot from that.

Any road, the shifting isn't going to get any better. It's just the way those cheap shifters are - I've had my low end Shimano equipped bike for 4 years, and it still drifts all the time, same with my g/f's bike. I knew this would happen though - the interesting part for me is seeing how long it'll be before the bearings start to go.