Some people do think that Lantus insulin causes some patients to gain weight, while the new Levemir insulin apparently does not do this. I read some articles about this just yesterday and was surprised, for I take Lantus and have noticed no weight gain at all. So this could be a factor.
However, a change in metabolism in middle age probably is the real culprit, along with possibly a slower metabolism because you are not eating. Exercise should help both problems. Start walking! Park your car further from the door, take stairs instead of elevators, etc. Or use your lunch hour to do some mild exercise right in your office. Weight training may be a good solution for you, for as you build up muscle, the muscles require more calories than the old fat cells, and so building muscle is a good solution. Some small hand weights may be helpful... you can use them at your desk when taking small breaks, or at home in front of the tv. I personally am not overly fond of going to a gym, either. I like to try to do the simpler ways of adding exercise to my day.
Funny, I posted a similar e-mail 2 weeks ago. But I have to say that insulin does cause the person to gain weight and age does not help either. I got that information from Kaiser Permanente's web health library. I began to gain weight when a was placed on 4 shots per day. In addition, I also have to take neurontin which also has weight gain as a contraindication. So, inform your self by researching the contraindications of insulin as well as checking on all your meds. You should also talk to your doctor.