Always think in terms of minutes, at least that's my recommendation. A 20 minute 5ker would run 4 miles at a hard floating pace in around 28 minutes. 10 years down the line that same runner could have improved to 14 minutes and run that same workout in 20 minutes. It's not wildly different but you'd end up under training as you get better. I think it's more consistent to think: 20 minutes at x effort rather than 20 minutes at x effort (or worse yet, pace). Don't get overly anal about the details either, just keep it somewhat organized so you don't get lost.
Anyway about your original question, the best advice I've ever gotten from letsrun was from something Wejo said. Practice running relaxed. Seriously.
I used to go hard on tempos, not "race hard" but still hard enough where I wasn't running relaxed. You want to aim for a pace that's fast but that keeps you relaxed and in control of every movement of your body. After you get that down, duration doesn't really matter.
I used to do a 10 minutes slow as hell warmup and if I felt like I had it in my legs, just keep picking up the pace every 5 minutes until I couldn't relax anymore. The rate of pick up differed and so did the overall duration, but I'd guess I almost always ended up at around the same pace. I also used to long intervals, some days were 10 x 5, others 5 x 8, etc. Pretty much the same jist but with 3 minutes of easy running and no stopping in between.
That approach combined with super easy recovery running (40-75 minutes at a old mans) on my off days (basically days where my legs just weren't ready for any hard training) turned from a 15:40 bum to a 14:20 bum during my college years.
Just FYI, I took that relax approach to everything, even hard intervals. If my coach had me do 3 x 1600, 800, 400 - I'd go fast but not leg failing fast. Always in control of my body and rhythm. I ended up going faster in those intervals than when I used to force it anyway, so it couldn't have hurt me. I seriously think a lot of runners never figure this out.