Runner's World Training Predictor

Based on my 5K time of 25:46 this is what Runner's World predicts I can do :

1500m 7:11
the mile 7:44
3000m(3k) 14:59
3200m(about 2 miles) 16:02
5000m(5k) 25:44
8000m(8k) 42:22
5 miles 42:37
10,000m(10k) 53:40
ten miles 1:28:52
a half marathon 1:58:19
a marathon 4:06:41

And their recommendations based on that pace:

10:26 min/mile Easy run training pace
8:43 min/mile Tempo run training pace
7:52 min/mile Maximum oxygen training pace
7:17 min/mile Speed form training pace
10:26 - 11:45 min/mile Long run training pace
4:02 min/800 Yasso 800s training pace

What I don't get is why they want you to run your long runs that much slower. How do they expect you to run faster if you don't do it in training? Especially for a marathon. There's no way. I have only trained to 20 miles on all my marathon training plans (usually doing one or two 20s). So speeding up for the race AND adding a 10K to it just seems like you'd end up with a really bad day because of elevated expectations. What training regimen do they base this off of?

And my 800s are a lot faster, like 3:30min/800. All distances below the 1/2 mary, I have done...if not faster. But I still haven't broken 4:30 in a marathon, let alone run a 4:06 (which would be AWESOME if I could). Makes no sense to me......

What got me interested in this is today, I did my second 10 mi at half-mar pace. I'm sick with a cold and it was really hard to keep my average down, but if I kept that pace it would be a 2:00.15. Once I recover from this cold and jet lag, it should be good. I hope. Whatever happens, I know for sure, that I will earn every second of that new PR. PR or die trying anyways, we'll see.