It could be wrong diet, lack of rest, poor training system, etc... but right away it sounds like overtraining. You're working HARD 6 days a week. Some professional athletes will only do that 25 weeks out of the year, and will snack on burritos for the other half. How long have you been doing that routine?
PS: I don't love the sound of that isolated body part split weight training schedule. Especially the legs being all alone. These days many like to focus on more most-of-the-body exercises...
Try taking one month off ALL of that. Do something totally different.
One idea: try high intensity interval training three days a week. Do tabata intervals as hard and fast as you can without worrying about or manipulating the scoring system like the crossfit geniuses do. Just try running almost as fast as you can for 20 seconds, then rest (walk) for 10 seconds, then repeat this 4-8 times until your distance really starts dropping off. That will take like 10 minutes out of your week, and after a couple weeks of that you might just see a difference in your original test performance...
For that month only, you can try taking 5g creatine with protein and sugar/carbs and water an hour before each workout, and then make sure to drink more sugar/carbs afterwards. This can give your muscles more oomph to meet the goal after the month.
Wow. Thank you so much for this thoughtful advice. I'm cutting/pasting this to save for further consideration.
I've been doing the running program since January. The lifting program started about a month ago. Before that, I was an avid crossfitter for about 2 years. But I started to feel like XF doesn't do enough strength work. I'm convinced that, to be an elite crossfitter, you should come to it with a lot of strength. So I wanted to go back to a more traditional strength-building routine and work towards building a higher single-rep max for key lifts like backsquats, deadlifts, and overhead presses. So, I did the 3-way split with a 3x10 rep scheme for 3 cycles, now I'm doing 3x8's for 3 cycles, working my way down to lower reps and higher weight to see how high I can go.
I'm already (I think) doing high-intensity interval training 3 times each week. I think that's why I've gotten so much faster than I was back in January.
I don't know anything about supplements. I try to watch my diet and get as much protein as possible, but I'm not methodical about it or anything. I know that's a key area for improvement, but I just can't stick with it. I did the Zone for several months and saw good results, but I just can't seem to sustain it.