I am also 41 years old, married with 2 kids, and I had non- hodgkins lymphoma 14 years ago and was treated with radiation.
Since then I put on alot of weight and could not take it off at all.
This year my hair also started to fall out so I asked my doctor to check the thyroid. She said I was fine (the lab ranges are high - s/b below 2). I went to an endocrinolgist and found out it is indeed my thyroid. Apparently 70% of people with head and neck radiation get hypothyroid. Anyway - I am being treated and feel great. Finally losing weight. The terrible fatigue is gone.

Anyway what I'm saying is get your thyroid checked. It controls the metabolism. And good luck!