I've always been a slow runner. And I know I used to be slower! When I first started out, after completing C25k, I remember meeting up with friends at the gym and running on treadmills, setting it to a speed somewhere between 4.2 to 4.5 - i.e., around 13:30 to 14:15 minutes per mile - and that was not a "easy" pace (I had no concept of doing "easy runs" at all, at that time). On a scale from 1 to 10, it was probably a 7 or so. When I did my first 5k race, I was impressed that I ran the whole thing (no walking breaks), but it definitely a hard effort (maybe an 8 out of 10?), and it was at a 12:58 pace.
And I know I was improving - at least in shorter distances. My 5k has gradually, but consistently, improved. But for longer distances - i.e., half marathon - once I broke the 3 hour mark (yes, that was a hurdle for me, for the half), I haven't improved as each race as I would like.
Here's a half marathon training cycle from 2013:
The vast majority of runs are around 11:30 - 12:30 - but not because a 12:00 minute mile was super easy. But that's not what I could keep up from a long run or race! Ignoring the fact that I only got up to 8 miles for my longest long run, I only slowed down about 30 - 45 seconds for those long runs than I was doing normally. I was continuing to run virtually all miles at about a 7 effort level. And you can't sustain that effort for an entire half marathon (at least not when it takes you 2.5+ hours do a half - maybe if you're finishing it in 1.5 hours, you can maintain that kind of tempo effort).
I admit I don't know all the science behind this concept (so, yes this whole post is "I don't know what I'm talking about but I'm trying to apply it to my running anyway"), but my understanding is that your faster runs and your slower runs are primarily working two different systems: fast runs are anaerobic, and slow runs are aerobic. It's practically like two separate sports, in some ways. All my weekly mileage was anaerobic, so I couldn't keep it up for an aerobic activity like long-distance races. I hadn't developed my aerobic system.
Here's a few weeks last summer when I started doing this aerobic base training:
Around mid-week you can see some speedwork that's in the low to mid 12:00s, but the vast majority are in the upper 13:00s or lower to mid 14:00s. That seemed so ridiculously slow! But that's what pace I ended up running at in order to keep my effort level and breathing at what felt "easy" (i.e., what I felt I could sustain "forever"), and what I think is actually an aerobic effort level.
And here's this past fall, a half marathon training cycle.
This wasn't a PR race, and was basically right where I've felt my half marathon pace has stagnated. But - on a hilly course, whereas most of my halfs have been on very flat courses. So, improvement in effort level to achieve that pace? Maybe. In both race day and training, it's hard, sometimes, to see small improvements versus mere variances in wind, temperature, caffeine, etc.
But here's what my running has looked like so far in 2015:
Now, I feel like not only has my speedwork gotten better (easily averaging in the 11:00s, even counting a warm-up and cool-down - the 10:50 average was a tempo that I didn't actually include a warm-up), but an easy pace on moderate distance runs is now down into the low to mid 13:00s! I've only just in the past couple weeks re-started doing a real long run, and I do feel like I'm seeing benefit here! Though keeping the pace slightly faster than easy, it's definitely not a hard effort (maybe 5 out of 10), and I'm keeping it right around 13:00! And that 13:00 is what I'm used to doing during races for the half marathon - if I can continue training at that speed, I'd better be able to race at least a minute per mile faster.
There are of course, other factors: I've lost a little weight, I'm eating better (which is part of the losing weight, but also just a higher quality fuel), I'm doing weight lifting (building stronger leg muscles in a way that running itself doesn't build, but can utilize). Maybe those are all more, or as, important factors in running feeling easier. But this is also the first time I've truly focused on keeping the easy runs easy and the hard workouts hard.
But whatever the cause, based on my running so far this year, I've fine-tuned some pretty ambitious goals for the year. I really really really want to break 2:30 in the half, and I'm now in the first week of an 8-week training cycle for the first attempt at that. Maybe I'm still totally delusional in what I think is possible, but you won't know until you try, right?