Muscle Fatigue and Endurance

I need some advice. I am 30 years old and swam in high school and a year in college and am getting back into it again. When I race now or do hard sets, I have no endurance at all despite being in relatively good shape. I realize aging affects me, but how can I build up endurance where I'm not in complete pain and fatigue at the end of a 50 yard free sprint. I'm getting oxygen but does anyone have a suggestion of how I can build up some endurance and not want to stop after 25 yards? ANY advice would be appreciated. Thank you!!