This energy replaces the energy that is radiated away and keeps the Sun from collapsing. Suppose the Sun had twice as much mass as it actually does. Obviously, the center of the Sun would be under greater pressure, and, through the equation of state, the temperature at the center would be greater. Greater temperature means faster moving particles. The Sun would be brighter.
Nuclear reactions can continue as long as there is hydrogen to fuse.
The Sun has been fusing hydrogen to helium in its core for 5 billion years, and will continue to fuse hydrogen for 5 billion more years. However, eventually, the hydrogen in the core will run out. There will still be plenty of hydrogen outside the core, but this will do the Sun no good. When the core runs out of hydrogen, the structure of the Sun must change. When the core runs out of hydrogen, gravity will begin to take over and the center of the star will slowly collapse.