Can you Help me swim the 500 scy in the 5:50s? Workout tips please.

Hi, I've been reading a lot of what you guys have to say and some of the people on this forum are really helpful. First off, I'd like to say that I have TREMENDOUS respect for good long distance swimmers. I know what it feels like when I do a hard fast 29-33 second 50, and I can't imagine keeping that pace up for a 500...much less a 1650. I was wondering if somebody could give me some workout tips for the 500 free (SCY). I'd really appreciate any insight. Times (SCY): I set a personal practice best last week with a 6:18 off the wall. I think I split (rough estimate from glances at the pace clock) 1:09 1:14 1:19 1:19 1:17. My PB in 2001 was something like 1:05 1:14 1:17 1:18 1:15. How do I reduce my fatigued race pace to anything around 1:10-1:12? Even when I try to take out the first 100 really easy, I still have terrible 3rd-5th 100s. Is it all mental, do I just need to bring it home tougher and sooner? I've been working out 4 days a week about 3000-4000 yards doing things like 10 x 100s on 1:45 coming in on 1:15ish. I can sprint 50s off the wall in 28 or 29 on the 3:00 and I sprinted a 100 in 1:04 at the end of a hard grueling practice. I can complete a set of 10 x 50 frees on the :55 coming in on 34-35 every time. Should I just keep trying to complete the same set with less rest? I also tried 5x75 on the 1:30 followed by a 125 and I held a 36 second 50 pace the whole time which would equate to a 6:00 500 free. I'll reiterate that I have tremendous respect for those people who can hold such fast long distance paces. Thoughts?