More likely is that the OP just isn't doing enough. Running 25 miles a week is enough to bring about significant benefits in health and fitness along with moderate weight control benefits. 25 miles a month is just a waste. Going up from 1-2x per week to 3-4x makes a huge difference.
Most likely is that it's a dietary issue. While living in Asia, I knew many, many non-exercising people at healthy weight levels just because they didn't overeat like Americans tend to. The OP probably doesn't eat like them.