So far I've read that it is about four miles of running to about one mile of swimming. I know the exercises are quite different, but aerobically, what is the equivalent?
Look at 200 metres swimming word records for both men and women. Look at 800m metres world records for both men and women, T&F. Four to one is the correct answer. There are posters on this thread who do not accept the 4 to 1 ratio. Due to temperment and body build, most individuals are not psychologically and are not physically able to do well at both running and swimming equally.
List of 200m world records, freestyle swimming: men long course, 1:42.00, short course, 1:39.37; women long course, 1:52.98, short course, 1:50.43
List of 800m world record, T&F: men, 1:40.91; women, 1:53.28