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.
Distance may range from  mile to 1.5k or 800-1700 yards. Swim distance is 1.5k or approximately 1700 yards. Swim distance is 1.2 miles or approximately 2000 yards. Swim distance is 2.4 miles or approximately 4000 yards.