This means that the battery is discharged down to 10.5 volts over a 20 hour period while the total actual amp-hours that it supplies are measured. Sometimes ratings at the 6 hour rate and 100 hour rate are also given for comparison and for different applications.
Pool meets are usually no more than 4 hours, but beach meets last about 7 hours. It depends how many events are in the meet. And then if it is a really big meet (like a tournament kind of) it can last days. Hope that helps.
The Ironman swim distance is 2.4 miles. If you can swim one mile (0 to One Mile), it is easy to increase to 2.4 miles without unusual effort. Beginning with a straight swim of 1650 yards or 1500 meters every Monday for a month, + two swim workouts of the same distance per week, increasing by 500 meters per month, it will take 6 months to reach 2.4 miles.