Some call it the "middle age slump" or "weight creep". You may not be doing much different than you did 5-10 years ago, yet the pounds are increasing a bit each year. As much as you don't like it, this is natural. In fact, thought to happen for survival purposes as a store of energy in case one should fall ill. As part of this natural course there is a shift in our body composition, the loss of lean body tissue, i.e., muscles, and often the gain of fat. Fat tissue equals energy stores.
As we lose muscle, we lose our metabolic machinery. Muscles need more and use more calories than fat, even at rest. Thus, we could be burning 100-150 calories less each day simply by getting older and having less muscle. That may not sound like many calories, but day after day, it adds up and so do the pounds.
Regular strength training counteracts loss of lean muscle mass due to aging. Consider adding strengthening exercises to your routine, perhaps 2-3 days per week. Talk to an experienced exercise professional to discuss the best options for you.
Get out of the slump, burn more calories, and age well.