THE beginning of October marks more than a seasonal change - as the northern regions of the United States shift deeper into fall. As of October the nation's economic expansion, now in its 59th month, becomes the longest period of continuous growth recorded in peacetime. Nor is the end in sight; the government's index of leading indicators, released earlier this week, posted the seventh consecutive increase. The economy continues to expand modestly. Politicians, economists - and statisticians - are understandably competing to take credit for, or explain, all this. For the moment, it is perhaps enough to note what has been a remarkable achievement for all the American people. Over 13 million new jobs have been created since the last recession, which ended in late 1982. Yes, many are low-paying service jobs. But they are new jobs. Still, Americans would do well to remember the other side of the ledger: About 7 million Americans remain unemployed. Another 5 million to 10 million work only occasionally, even though they would like to work more. Equilibrium is called for. Congratulations are in order during this 59th month of expansion. But the cheering should not come at the expense of ensuring that all Americans who want to work can do so.