The advantage faster swimmers have over slower swimmers is technique
Now you have just read the previous post about strength and conditioning, your gym bag is packed and your about to go for workout, but wait there is more!
Research by Havriluk (2010) Performance level differences in swimming: Relative contributions of strength and technique found that technique contributes more to performance than does strength; i.e. large gains in velocity result from small decreases in drag, such that even the fastest swimmers could benefit from improving technique.
Bugger! Now don't put away the gym bag, but don't forget to pack your swim bag with all your toys and get down to training on Tuesday and Thursday night!