the 5 minute 500

I always thought it would be cool to be able to break 5:00 in the 500. While it would be a bit of a stretch I feel like it's potentially within reach and given enough of the right kind of training I could do it. That said I also feel like every second between me and the 5 minute mark is going to get harder and harder to knock off. I've always been, and continue to be, more of a breaststroker than a freestyler. I have no ankle flexibility so I pretty much always need to deal with dragging two brakes through the water. I've never really trained seriously for free events even though I do try hard and consider the longer frees to be my secondary events. To give you an idea I swim about 3000-4000 a day, probably realistically average 4-5 times a week. I can and will increase that over the winter as we head into the meets starting in January. Intervals average in the 1:15-1:20 range, sometimes dipping to 1:10. I can pretty comfortably hold 1:10-1:15 pace depending on the set. Doing a 1:00 100 from a push is hard but not impossible. So far my fastest masters times in the 200 are 1:55 and the 500 a 5:18. I know I can go much faster in both, in that 200 I took water instead of air on the turn at the 100 and was choking and coughing for the last 100 and still somehow managed a best time. The 500 was the last event of a long weekend of swimming and I was exhausted. I bet I could have done a 1:53 and a 5:14 given better circumstances. Anyway, any training tips or things to work on? Ideal way to split it? Is the 5:00 500 going to be ridiculously difficult or given enough training reachable? Thanks guys. That and the :50 100 (I'll save that one for later) are two swimming marks I always wanted to check off my list.