I think it is all about core strength.
Even though I ride all year round, 2-3 hour rides, 2-3 times a week in up to -30C conditions in the winter, I gained more pounds than I wanted over the winter. Also a herniated disc has been acting up the last 3 months or so.
So it has only been the last 4-5 weeks that I have been riding on a pretty well daily basis of 25-35 miles. At the beginning I noticed my hands went numb fairly soon and often. However the more I rode the lighter my grip seemed to be on the bars. I am sure this is because my core is strengthening (and I lost 15lbs, but am still way too heavy). I really make a point now of having a very light grip on the bars, almost barely touching them, just enough to have control of the bars. I thought this would put more stress on the back but it seems once your core gets stronger it is a very natural position and easy for your body to be in this position, even for 2 hours.
Even though I ride quite a lot, I am tempted to try core specific exercises to see if it adds significant improvement.