Im not 40 yet but close so Ill post here.
I have been a wrestler, a model and an all round sportsman, up until a couple of years ago I had no weight problems to be concerned about I could put on or take off 10kg in a short period of time, plus I trained all natural.
Along comes a injury which put me out for a year completely and I put on weight, alot of weight. However whereas before I would just bounce back, this time it didnt happen. After over a year of training I began to ask myself whats wrong. Im 125kg and those 1 hour long runs that I used to love needs serious step by step efforts to achieve. Strength is not a real problem not at this age as yet anyway.
Anyway to the point, I start to consider testosterone as an aid in recovery, joined this board and began to read up on whats going on over here.
What has become apparent is that Ive reached and am headed into middle age, it has become time to reconsider my approach to my spiritual, mental and physical health. Eating what I want when i want is no longer possible, now at this age diet has become a even more important. Eating large sugary meals throughout the evening is a definate no no for me. In way I have come full circle, I used to really care about my diet and it got me into the lifestyle that I became accustomed to. All I had to do was to maintain.
Now the metabolism has slowed and my testosterone beginning to decline and it really has taken its effect, I have become FAT.
In my quest to come back and show everyone that I can do it again and this time with the aid of testosterone, all of the material points to what I have experienced above. It is very shocking and enlightening at the same time to find out that testosterone plays a major role on metabolism, insulin levels, and fatty tissue. Lack of this hormone really can take its toll on the body. The other incredible thing is that the fatter and older you get the less testosterone your body wants to produce. If however one drops the weight, more testosterone is released. This however is a catch 22 situation, as I have found out. The body fights any change and desperately wants to stay as it is, finding the motivation to train,lose weight, and eat right takes some serious motivation even for the seasoned athlete. Where is the testosterone?
This is where I have to say that the introduction of the synthetic hormone testosterone has improved my motivation. It has done this by raising all those levels of metabolism, fat burning and my bodies ability to fight diabetes, (which I dont have yet). It has also had an indirect effect but in a no less profound way, through the learning curve that I have had to go through to understand how the body works in order that I use any substance, safely.
Anyway I just wanted to get this out as it has been a struggle to understand and cope with the "middle age spread".
Yes it really is true life really begins at 40!

Great post, very sincere.
Don't be so upset by the fat you weight you are adding, take it as a temporary and resolvable problem.
Maybe it is a more hard to get rid of it but anyway it is possible if you work hard.
Good luck.