(1) For a new runner, I think a good goal would be to average under 7 minutes per mile for a 5k. 6:59 pace x 3.1 miles = 21:39.
(2) After that I'd imagine 20:00 is a nice barrier. To run 19:59 you'd need to average 6:26's.
(3) 18:59 - now you're starting to get a little more serious. 6:07 pace. That's not easy, many people cannot run one 6:07 mile, much less 3.1 of them.
(4) 17:59 - now I actually think of you as a competitive runner. Well done. Before that you were just a hobby jogger to me, but I see that 17: out front and I know you are probably doing workouts, running 5-7 days a week. 5:48 pace.
(5) 16:59 - now you're a pretty good competitive runner, you are coming in top 15, 10, or 5 in local road races. 5:28 pace.
(6) 15:59 - you're ballin'. 5:09 pace. Normal people find these speeds shocking, and for good reason. They struggle to run 6-flat for one mile and you can destroy that now. Here's the problem - once you're running sub-16 it's like you've graduated. In the 16's you were the fastest of the slow people. Now that you're in the 15's you're the slowest of the fast people. It's like going from 8th grade to high school.
(7) 14:59 - 4:50 pace. Awesome barrier to break, but you're kind of in no man's land. Local road races are too slow for you now, they offer no competition, so you'll have to find college or high level open meets to keep challenging yourself. You rock, but you're still a very long way from that Trials B qualifier.
(8) 13:59 - 4:30 pace. In the running world, you are now a big fish. Granted, you will get destroyed by top Americans, but they almost don't seem real. They're just guys you see on TV and read out in magazines. If you break 14 you are a legend in your local community of runners. Nearly a Trial B qualifier (13:50). Nice.
(9) 13:30 - 4:21 pace. By now you are one of the top Americans in the sport. Well done. Olympic Trials A standard. You're ridiculous. Good for you. I am jealous.
(10) 12:59 - 4:11 pace. 4-freaking-11 pace. Jesus, people are capable of that? You're top 15-20 in the world.
(11) 12:37 - 4:04 pace. Too impossibly fast. Fastest human of all time. How can someone even run three 4:04 miles in a row?
In conclusion, you should try to run 12:36.