I'm thinking of going swimming in the morning before work a couple of times a week on my easy running days. Does anyone know if it's good cross-training for running?

I won't be flouncing around doing gentle breaststroke, I'm a good swimmer and I'm hoping to do 100 metre front crawl intervals pretty hard. I'm assuming this type of swimming would improve your aerobic capacity as much as running, but give your legs a rest and improve your core strength.