Here is a quote from the US Military history on the ARW. Note that new militia from New Hampshire and Massachuetts were called up to replace departing Connecticut militia at the end of 75. Also note that 8000 continental soldiers were enlisted by 1 Jan 76 with their enlistments expiring in Jan 77. I am assuming most of those new continentals were militia that agree to join the continental army rather than go home as most did.
http://www.army.mil/cmh-pg/books/AMH/AMH-03.htm
While establishing discipline in the existing army, Washington had at the same time to form a new one enlisted directly in the Continental service. Out of conferences with a Congressional committee that visited camp in September 1775 emerged a plan for such an army, composed of 26 regiments of infantry of 728 men each, plus one regiment of riflemen and one of artillery, 20,372 men in all, to be uniformly paid, supplied, and administered by the Continental Congress and enlisted to the end of the year 1776. Except for the short term of enlistment, it was an excellent plan on paper, but Washington soon found he could not carry it out. Both officers and men resisted a reorganization that cut across the lines of the locally organized units in which they were accustomed to serve. The men saw as their first obligation their families and farms at home, and they were reluctant to re-enlist for another year's service. On December10, despite pressures and patriotic appeals, most of the Connecticut men went home and militia from New Hampshire and Massachusetts had to be brought in to fill their places in the line. Others, who had jeered and hooted when the Connecticut men left, also went home when their enlistment expired only three weeks later. On January 1, 1776, when the army became "Continental in every respect," Washington found that he had only slightly more than 8,000 enlistments instead of the 20,000 planned. Returns in early March showed only a thousand or so more. "I have often thought how much happier I would have been," wrote a sorely tried commander, "if, instead of accepting a command under such circumstances, I had taken up musket on my shoulder and entered the ranks, or, if I could have justified the measure to posterity and my own conscience, had retired to the back country and lived in a Wigwam."
With enlistments falling short, the only recourse was to continue to use short-term militia to fill the gaps in the lines. A Continental Army had been formed, but it fell far short of the goals Washington and Congress had set for it. This army was enlisted for but a year and the whole troublesome process would have to be repeated at the end of 1776. The short term of enlistmentThe short term of enlistment was, of course, a cardinal error, but in 1775 everyone, including Washington, anticipated only a short campaign.