+1 to the above and my own congrats for getting back on the bike!
Physical adjustments take time. Your body will adapt to the new activity; however, neck and shoulder pain are probably more a fit issue. If you used to ride alot your position on the bike *now* ought to be different than what it was 15 years ago, especially given your weight gain. Are you riding a road bike? If so, the drop from saddle to the bars may be too great and this would put too much pressure on the neck, shoulders and back.