Sprint Sets- actual time vs workout target time

Well: As most of you may have guessed, I am doing sprint sets to get faster. I was doing the HACKET sets. I found that I could not hit the 45 seconds per 50yards (swimming 200 yards), and then move into the next 200 without stopping for 20 or 30 seconds. My question is, should I go slower ( take a 20 to 30 second hit on my 200's in practice)? Should I rest 20 or 30 seconds and then continue the workout?