Cool, good luck on your TT.
With your explanation on the 6x400 uphill/downhill, I might suggest the following as a predictor workout: 10 x 400 all uphill, unless the hill is a bit steep, in which case 8 x 400 uphill. With a 2 minute jog-back recovery. When you can do every repeat at sub-1:15, you should be able to go sub-5.
Here's why I suggest this as a better predictor workout:
1) 6 x 400 is a fine workout (esp if you are doing 30" rec as you did) but 8 or 10 repeats is generally considered better for mile training, broadly speaking.
2) Uphill repeats are great for power & strength development. Most runners would benefit from some hill workouts, meaning uphill repeats.
3) I always stayed away from downhill repeats. Unless done carefully as part of XC training, the wisdom I received was that downhill repeats aren't a great idea.
However, having said all that, I don't think that I personally would do the predictor workout. It's certainly not a horrible idea, but if I were in your shoes I would try to do the workout that would benefit me the most, in terms of likely improvement.
Personally, I would do 8 x 400m uphill with 3 min rec, and I would try to hit 1:10-1:12. That way, I would be getting the most out of the hill (power/strength) plus I'd be practicing running faster than goal race pace -- meaning that 4:59 pace will feel easier and smoother.
By itself, this workout wouldn't be much of a definite "predictor". But if combined with some 4 x 800 sessions @ 2:25-2:30 with 2:30 recovery, for me those workouts would complement each other well. Add in an occasional TT and plenty of aerobic runs and there you go.
Of course, there are many approaches and some folks would disagree with the type of approach that I suggest. As long as you have a well-thought-out plan that covers all your bases and is working for you, that's the idea.