The following was written by Weldon Johnson, founder of letsrun.com. I think it makes a lot of sense:

First off, I'm not big on the scientific terms. That's why I turn down a lot of request for people to coach them over the internet. For me to scientifically design a running program with specific paces would be difficult. Even more difficult would be for me to explain things scientifically. But I think a lot can be learned from this. First of all, if you have a coach who could design workouts for you that you trust, it makes things a lot easier. There is no doubt in your mind then. A workout is assigned you believe in it and go do it. If you're unsure of what you are doing and start questioning everything you do, then you won't have quite the confidence when you race. And the one thing you must have when you race is total confidence. But not overconfidence... But turning back to the question at hand, as I said I'm not big on the scientific terms or judging the pace off of a watch. I think the key is to listen to your body. That should be your guide. When I sucked at running, I trained off of my watch, when I was good, I just used my watch just to record my times. I think one of the reasons I made such drastic improvement when I moved to Flagstaff (editor's note: his 10k pr went from 29:49 to 28:27 in 1 race), besides the altitude and increased mileage I was running, was for the first time in my life I had to just listen to my body when I ran. I'd go workout on a dirt track at moderate altitude that makes the tracks in Kenya look like Mondo tracks. I'd ask my coach, "What pace should I run". He'd say he didn't know, he had no idea with the altitude and surface involved. He had always described things in effort, but given me a pace as well. However I always focused on the pace which was a mistake. For the first time, I just had to go off of effort. And soon my watch was almost just a recording device for me. 25-30 minute tempo runs are probably my favorite workout. Now sometimes, these are what I call, "high steady state" (no emphasis on pace, I might even run longer say 45 minutes), sometimes they are true tempo runs, and sometimes, very rarely (especially if I'd got a little shorter in distance), I'd feel the need to try and hit as close to a certain pace as possible (and that was when I was at low altitude). The final instance was the only time where I would actually use my watch as a guide during the workout. For this, I have a 5-10 second range in my head that I knew I needed to start off at, and since there was an emphasis on pace, I wanted to make sure I was starting off ok. But I would exit the track after 3/4 of a mile, then do 2 road loops off the track with no mile markers, and then finish with say a faster mile on the track. But for the majority of these runs, all the watch would do was record my loops. Usually the only time I looked at it was the times I started off on the track. Oftentimes, I NEVER looked at the watch during the run. I would just hit the "split button". I never measured the distance of the loops. I'd just report the times back to my coach. This may sound crazy, but it taught me to learn how to listen to my body and how to run fast. One of my favorite things was finishing workouts (I did this same thing for a lot of track workouts as well so don't think this only applies to road stuff), and then looking at the split times, to see how the workout "went." And things almost always went well (actually without a watch I think it is really hard to have a bad workout) because I was running how I felt, not trying to force an artificial pace (don't get me wrong there are a few times you really need to try and hit a pace but they are few and far between). And basically I'd evaluate these runs on one criteria, whether each lap was faster or as fast as the same one before it. It never ceased to amaze me that EVERY single time I worked out (at least that I remember), this was the case, until the workout I blew out my plantar and kept running on it. (I got back that day and was like "Whoa my foot must be messed up, I just ran that loop 30 seconds slower). Even days when I felt like I was slightly struggling, I would not be slowing down. And that is because on most tempo runs (I'm using that term broadly) you don't want to be over your threshold until the final mile or final 3 minutes. My plan for most of these things was to perhaps start out a little slower the first mile, exit the track, be able to maintain or pick up that pace, and then be able to bring down the pace for sure the last mile, but for sure the last 2-3 minutes. If you can not do this, you are running too hard too soon. If you are dying in a tempo run, you might as well not do it. I can not reiterate this enough. As Arthur Lydiard said, "Relaxation is the key to running fast." To do this, in almost everything you do, you should be finishing faster than you started. If I had one rule for running that would be it, Finish Faster Than You Start. And the absolute pace on my runs would vary tremendously. I remember doing say 4*8 minutes on this one loop and my loops were over 7 minutes with rest. Now this might upset some people because I knew the year before I had done 4 continuous loops in around 6:30 a loop. So here I was running 30 seconds a mile slower WITH rest. But this was where I was at and the pace I needed to run. The next week things would be faster, etc. But when getting into shape, you can not force a pace. Thus if I tell you to run 10% slower than your 10k pace I don't think it serves a lot of purpose.

I have a hard time with Wejo (Weldon's letsrun name) sometimes but this is really well said. I was thinking about this very subject during this morning's run. I went out to do one of my staple workouts and I was thinking about this particular forum and how the heck I would ever explain this type workout to someone, let alone over the internet. I'm racing on Sunday and I wanted my mid-week workout to be relatively easy so I went back to my standard 8 x 2 minutes on / 1 minute off with 4 x 30 seconds FAST at the end. I've done this workout a hundred times and about 98 of those I had very little idea how fast I was doing it--and it didn't really matter. Lately I've done that workout using my GPS, which does two things; it tells me without having to look at the watch when its time to go and when its time to jog, and it records everything so I can see later how fast I went, if I want to. Anyway the details are here, but to me there are two things I've learned from using the GPS for workouts like this; 1.) I generally always get faster as the workout goes on because I'm doing it by feel and not trying to hit some pre-determined pace from the get-go and 2.) my pace varies widely from week to week, but it's okay. Though my overall pace for the workout varies week to week, the pattern is nearly always the same--my first 2 minute "on" is about 1:30 faster than whatever pace I jogged the warmup, and each rep after that gets a bit faster than the one before it. Which means, to me that I'm getting exactly what my body can handle out of the workout each time I do it Today I was tired--I could tell from my warmup jog wich is normally more like 7:30 pace but today was around 8 pace. Then the first 2 minute on section was 6:35 pace--which is much slower than I was doing 2 MILE repeats a week and a half ago--but by the end I was doing them at just under 6 pace. Still slower than normal but its okay. The workout was a success and it accomplished its goal.