Bob Dole resigned from the Senate after securing the nomination. It was sometime in the summer, a little before the convention, if memory serves correctly. Plus, Dole served in as Senator in Kansas since the late '60s. And, let's not forget Dole was old, very, very old.
To draw some kind of comparison between what Bob Dole, just months before the election, and what Palin did years before any possible election, is a stretch, at best.
Yes you are right - the comparison actually favors Palin. Running for President is a full time job. If presidential hopefuls were honest with their fellow citizens they would have to acknowledge that running for the Presidency prevents them from fully focusing on their day jobs.