Eaton to a 3:45 1500m, and possibly sub 4 minute mile, is very much possible within 6 months. This is my perspective:
Currently he is in 47.x 400m shape or better. That's a logical guess given his recent hurdles performance.
I simply go up from there.
47.x means is capable of a 1:21.00 600m, most can agree on that I'm sure.
1:21.00 600m is capable of 1:57 800m, and most can agree on that I'm sure.
I believe it can be estimated with a high degree of accuracy that Eaton is in 1:57 800m shape. I believe these estimates are conservative, but for the sake of a 3:45 1500m goal in 6 months, this is where I'll start.
I'd give him 3 months of distance running as a base. Starting with 30-35 minutes of jogging, 5-6 days per week (25 miles per week) with a once per 10 days speed session of 5x100m nearly flat out, 8x150m hill at 90% effort, etc). Do this for 3 weeks. The next 3 weeks, up it 45 minutes. Now at 30-35 .
Now, move progressively to a peak of 40-45 miles/week with a once weekly fartlek session. During these three months, throw in 3 time trials or races. 1x400m, 1x800m, 1x1500m. Also, allow for a weekly day off, and the 6th day of training per 7 days to be optional: A recreational soccer game, swimming, cycling, or completely off.
After 3 months of this, test Eaton at 800m. With his speed being worked on throughout, I`d guess Eaton would be capable of at least his original 1:57, though likely closer to a 1:52 (or better)
Now, with a base of distance running, I`d simply throw in track workouts, staring with distance repetition workouts (5x1000m, 6x800m, 10x600m) at 3k-5k pace, while progressively moving to more speed oriented workouts at goal 800m & 1500m pace. (10x200m, 8x400m, 6x300m, etc). Through these next 3 months, I`d keep the weekly milage anywhere between 25 and 45 miles per week, depending on Eaton`s adaptability and recovery levels. A very low milage week (15 miles, for example) wouldn`t be out of the ordinary with 2 races, for example.
Within 6 months, I am confident that an athlete of Eaton`s talent level, while maintaining his 47.x 400m speed, and ideally improving it to his 45.7 personal best level, would be capable of bring that 800m down to 1:47, and tackling 3:45 in the 1500m. A sub 4 minute mile, in ideal conditions and with ideal pacing, wouldn`t be out of the question.