As you continue to practise, please understand: there is nothing to worry about. Establish this feeling of being relaxed and unworried, securely, in the mind. Once the mind is concentrated and one-pointed, no mind-object will be able to penetrate or disturb it, and you will be able to sit in the meditation posture for as long as you want. You will also be able to sustain concentration without any feelings of pain and discomfort.
Having developed samadhi to this level, you will be able to enter or leave it at will. When you leave it, it will be at your convenience. You simply withdraw at your ease, rather than because you are feeling lazy or tired. You withdraw from samadhi because it is the appropriate time to withdraw from it, and you come out of it at your will.
You enter and leave this samadhi without any problems. The mind and heart are at ease. If you genuinely have samadhi like this, it means that sitting meditation and entering samadhi for just thirty minutes or an hour will enable you to remain cool and peaceful for many days afterwards. Experiencing the effects of samadhi like this for several days has a purifying effect on the mind. Whatever you experience will become an object for contemplation. This is where the practice really begins. It is the fruit which arises as samadhi matures.
From The Collected Teachings of Ajahn Chah.