In answer to your query, several posts above:
I tried to make it as a walk-on XC guy my senior year in college and wasn't nearly good enough. Kept running and within a few post-collegiate years (by the age of 25) had run the following times:
5K: 17:27
5 mile: 28:00
10K: 35:49
10 mile: 59:35
marathon: 2:53:30
I quit training and racing in 1984 and, although I kept jogging (3-4 times a week, 3-4 miles, easy), I didn't run seriously, or race at all, until April 2003. My first 10K back was 47:10. My second 10K was 48:04. A week later I ran my first 5K in 22:00. That seemed like a reasonable baseline.
After several years of pretty dedicated training (40 mpw all year long, give or take a little), I've lowered my times as follows:
5K: 19:41 (I also have a 19:30 but I suspect a short course)
10K: 40:48.
Each season, as I've aged from 46 to 48, I've pared some time away from my old-guy PRs. I see the writing on the wall--no return to actual former time-ranges--but the verdict is still out on exactly how much time I can still scrape off those numbers. Obviously I'd love to hit or break 19:00 and 40:00. Those are good round numbers to guide my training.
Interestingly, as I get stronger over these return-to-running years, the races don't hurt quite as much. I think that's because I've learned the art of proper pacing. It's also because I've learned to push myself hard at appropriate moments in training, so that race-pace on race-day doesn't come as a shock. In fact, since I'm tapered and adrenalized and don't go out too fast, it comes as a positive relief: show time, folks! The wait is over. Show me whatch got.
The advice from other quarters about easing up slightly on your easy days and running your intervals a bit faster is excellent. Also, if you're running six miles, it's always a good idea to run the first mile--or at bare minimum the first half mile--pretty easy, without pushing at all, to give all systems a chance to come up to speed. If you're running a ten-miler, think of the first five miles as the warmup and the last five miles as the push. Think progression run, in other words. If you're in a summer base-training phase, actually, it's good sometimes just to go out and run easy/steady with no push for the entire 10 miles, letting the work be the simple fact of putting in the miles. It's taken me a couple of years to understand this principle, in my bones, but it's finally sinking in.
Of course sometimes it's fun to just go out on a training day and hit it hard, and have fun. Sometimes we get so rational about training that we forget about the primal side. Running is nothing if not primal--or can be--and it's good to honor that side of the sport, too.
. I imploded about a 1/2 mile later when reality caught up with me (and ended up not meeting the sub-20 on that attempt).
