Insulin is absorbed well from the layer of fat that lies under your skin. All of the estimates of the onset, peak, and duration of a particular type of insulin assume that the insulin was injected into this layer of fat under the skin. If you injected too deeply by mistake and the insulin went into a muscle that was under the skin, then the insulin would be absorbed much faster. This is most likely to happen if you do not have much fat covering the muscles on part of your leg. Muscles have a bigger supply of blood vessels running through them, and so injecting into a muscle is more likely to bleed. It also makes it more likely that you will have a low blood glucose level in the next hour or two.
So make sure there is a layer of fat below the skin where you are injecting.
The most common sites to inject are the abdomen, the thigh, the buttocks, and the upper arm. Some people find that the skin over their abdomen is a bit more tender than the skin on those other places. There is also a layer of fibrous tissue running up and down in the middle of your abdomen (above and below your belly button), so you should avoid that part. Insulin is absorbed slightly faster from the abdomen and slower from the buttocks and thighs (with the arm being in between). So if you find that your long-acting insulin is not lasting long enough when you give the injection in your abdomen, then switching to give it in your buttock might work better.