For anybody interested, I thought I'd share some of my philosophy/reasoning re OP's case. Anybody feel free to agree/disagree/ask questions. Maybe this will stimulate an interesting discussion about speed training etc, while we wait to see how OP does.
Sorry, this will be long-ish as I explain ... I'll try to be succinct.
I agree with blackloud's points, above. For these reasons, I did not suggest that OP include full-on top-end speed work like 30-60m repeats, or true speed endurance stuff like 150 repeats.
That's also why I suggested ditching the long run - that rest day would be important to letting the fast sessions have their effect. However, OP wants to give the 400m training a go for 2 short months, while still keeping in touch with LD, and then get back to more mileage. So, while a long run isn't going to help the 400 effort at all, and may actually hinder the training a bit, I get why OP wants to keep it - given his big-picture outlook.
I figured that OP would not benefit from just jumping in and trying to run fast. Starting with the longer repeats, working them down over time, and getting progressively faster would hopefully allow for an injury-free transition to running fast. Pushing each of those repeats to a good hard effort is, I believe, key.
The long intervals should start with the 1200 a bit slower than mile pace and get faster. The 800 should be at mile pace. By the time OP works down to the 600/400/200, he should be running the 600 around 1:40-1:45, at his estimated 800m fitness. The 400m repeat should be quick/smooth/relaxed, about 4-5 sec slower than 400m fitness (not goal pace, current fitness. The effort is based on feel). The 200 should be nearly all-out 98% effort. Hopefully, OP can hit 26-27, but after the 600 & 400 I don't think it would be terrible if he ran 28.
Regarding the 4 x 300m workout: I figured that 300m is too long for OP to strain something by running too fast too soon, so it's probably safe to push it as hard as he wants. This workout could quite realistically allow someone like OP to run with better speed if done repeatedly over time.
For example, OP has run a 57 400m and wants to go for a 54. 57 pace would be running the 300's in about 43, but who knows where OP is going to be for 300 repeats speed after all the LD training? Let's say he starts out the first session somewhere around 48-50 sec. That's fine - if he pushes it hard during each 4 x 300 session and if he takes the proper full recovery, then within reason he could get them down to 41-43 -- which, if accomplished within the context of the rest of the suggested program, should hopefully get him within 54-57 range.
Of course, 39-40 sec would be better for the 300m repeats but without plyos, 30-60m's, and 150's, this 2-month effort is not going to DEVELOP speed so much as it will allow OP's basic natural speed to emerge, however fast that may be. I am thinking that this approach should be sufficient for a sub-57.
The program I suggested for OP is certainly far from a normal or ideal 400m regimen. The idea was to safely and effectively transition an LD guy to a mid-50's 400m effort - without overloading on the speed work. If he ran 57 last year being unfamiliar with 400m speed training, I figure he can go a bit faster by introducing the workouts I recommended.