Very good thread going here.
One more point about variability of paces. I was just out running and wanted to get a bit of quality work in. I'm coming off a down period after the Boston Marathon and my local race series starts with a 5k in August, so I'm basically at the end of my offseason/the start of my preseason. Went for a six miler, starting with an easy three then went a mile at roughly marathon pace (5:43), a mile at roughly tempo pace (5:31), the. A mile at roughly 10k pace (5:17). Averaged 3 miles at about tempo pace, but varied across a fair range of paces in the process. Not only did I get a decent threshold workout in, but because I wasn't really sure what my current fitness is, I got to shift gears a little until I could figure out roughly what my tempo pace is.
This sounds like the antithesis of what Daniels would advocate if you don't actually pay attention to the text of his book, but is thoroughly in bounds in a Daniels based program, and I particularly like this early and late in a season.