Hit the wall today in practice. Do you eat before you go.

WOW I am tired. I hit the wall early in practice today but kept plugging along. I am a 41 year old female who has been out of the water for 22 years ( competitivley). I have been told that is common and it will get better. How long to build up ones endurance and when does it get easier, though I know you will always have to push yourself. I just see some of the longterm swimmers glidding through the water like it is sooooooooooooooooooooooooooooooo easy. I must admit I am swimming much better than I thought I would be for how long it has been. I am swimming at 5:30 am. Do you eat before you go? and if so what. I also have a bit of a lower calf cramp. Tomorrows practice should be something :thhbbb:. But I am determined to do this. I need to get in shape lose some weight and just really love the swimming. SIGH Katie