I am not targeting an ironman, but I think I agree that you should swim the full distance (4225 yards) before the race. I mean, its not that far compared to the bike or run so it wouldn't come close to your longest workout. Also, from a safety standpoint, you have got to be able to swim the distance very comfortably.
I could be totally wrong... but coming to tri's from a swimming background, I have done twice the length of an ironman swim in a single workout training for 50 to 200 yard races. I know the training distance:race distance ratio shrinks as the races get longer, but for an hour swim, I'd probably make my longest training swim at least that long.
Your plan sound fine to me, but I don't really have anything to compare it to.... Now that I"m done with my swimming days and just working on olympics and sprints, I swim 3x per week. Twice I do about 3500 yards and make sure to incorporate some speed work as for that distance it is the limiting factor... The third time, I do a shorter workout that consists almost entirely of stroke drills (no matter how good you get, you can always fine tune something) -- although I through a hypoxic set in at the end.

Panchotri,
Michael Phelps at the top of the elite end may swim up to 7000m every day, and if he did it as a steady long swim he'd do it in just over an hour.
For the sake of comparison I'll tell you what I swam in the 8 months of training leading to my 1:22:00 swim in IM Canada 2003 at age 51 as a Clyde. It may encourage you.
Make of this what you will.
The IM leg in 2003 is the only time in my entire life I have swum more than 2Km in a 24 hour period. The week prior to IM I swam 2k each day, in the lake at an easy pace, no wetsuit.
The first time I'd ever swum in a wetsuit was the day before IM to test out the loaner I got the previous day from a friend.
My swim training began Xmas week 8 months before in a 25m pool. My January total was 7500m.
February I went to Puerto Vallerta. Mostly I just ran and biked but I played in the water every day to relax. February total was 3600m.
March total, back in the pool was 13,200m.
April, May, June I did not swim at all. I was concentrating my training time on the May 18th Penticton Marathon.
July I got into the lake. I'd ride 20-70K, run to the lake and have a swim. No pool work. July total 4000m
August was nice. I totaled 12000m in 3 wks prior to the IM Aug 24th, by swimming 2k each day in the lake for 5 days the week before IM. I didn't do any swim training in the 7 days immediately before the race.

In 8 months prior to IM my total swim training was 50,000 metres. Mike Phelps swims that every week.
My IM swim was a breeze. I was last into the water intentionally and swam my first 3.84km in my life in 82 minutes.
A long time ago, 30 years? I was a swimming teacher but have never swum competitively. I have had a technically sound stroke since back then so I am comfortable in the water and as a Clyde-sized person am not intimidated by the crowd in the water. I played water Polo in college, that helps.
I am not reccommending this as a program. I am telling you this so that you can compare the two extremes of swimming preparation for IM and find your own level of comfort in relation to your goals.
Develop a sound stroke and you shouldn't feel like you must swim 20,000yds a week just to finish IM. I think swimmers are horribly overtrained personally.
To put it into perspective, If I want to swim just 15 minutes faster, 1:07:00 for 3.84km, - that's 23 seconds per 100m faster, I will probably need to DOUBLE my training distance from 2003.

all the best with the training,
PoC
Oh, if you want to read my 2003 IM training log it is here: 2003 IM Training