I started running at the age of 15 when I joined my high school cross country team in 9th grade. The first day of practice during preseason in august the coach had us run a time trial of our home course. It was a mixed race with both the boys & girls on our team starting at the same time. I finished dead last in something like 29'40 or so. My time was just under 30 minutes. I was well behind the 2nd to last place finisher. By the end of the season I was the slowest on the team but improved my PR to 22'39.
I loved running but knew absolutely nothing about training and I didn't have anyone around me that did either. We had no track team so the only time I could compete was in the fall. Additionally my school had a rule where students had to participate in multiple sports each year, and with no running option during the winter or spring, I didn't run year round.
By the end of my sophomore year I improved my PR to 20'22. Keep in mind the training we were doing was laughable looking back on it now. Maybe 30-35 miles per week. Not difficult at all. My main improvements came from getting older. I did run occasionally during the other 3 seasons but it was very inconsistent and was never a "workout", only something like 30 - 60 minutes easy, which for me at the time was slower than 8 minute mile pace.
By the end of my junior year my PR was 19'42. I had broken the 20 minute barrier! I was happy but still very much a mid-back place finisher in most races. I wasn't happy about that and wanted to improve. By the end of my junior year my high school sweetheart had broken up with me and I was pretty bummed out. I decided to deal with it by running more so I would have less time to think about it. I started doing 3 laps of our very hilly 5k XC course every afternoon at a hard (for me) pace. I ended up coming down with an injury that would knock me out for 8 weeks.
Since I couldn't run for 2 months I decided to look for ways to improve without running so that I would have a head start upon my return. I started doing core work every day. Push ups, crunches, pull ups, planks, etc. I also made an effort to get my weight down. I'm 5'10.5 and was in the 150-155 range. I got my weight down to 140 all during the time I couldn't run.
Upon returning to training it was very difficult! I remember my first run was 5 minutes out & back and it damn near killed me. But over the next month I kept doing easy runs of this nature. Additionally I lost a few more pounds so I was now about 136-137. After about 5 weeks of easy runs I was able to start "real" training.
I began a 6 week period of training that was harder than I had ever trained before. Much of it was stupidly hard but I didn't know any better at the time. One week I remember went like this:
Monday: AM) 5 miles PM) 4 x 1600 at 5k pace
Tuesday: AM) 4 miles PM) 13 x 250m hill sprint at maximum effort, jog back & immediately start the next one
Wednesday: AM) 5 miles PM) 12 x 400 at mile pace
Thursday: AM) 4 miles PM) 20 minutes of 100m sprint / 100m easy
Friday: AM) 5 miles PM) 8 miles
Saturday) PM) 5 mile tempo run
Sunday) I skipped this day because I was fried
I also did a 10k "tempo" run on the track where I ran the first 5k in 18:29 with the first 2 miles under 5'50 and finished with a time of 37'47. This at a time when my "official" PR was still the 19'42 from the previous fall. The average mileage during this time was my highest ever at around 55 - 65.
I ran a 5k road race (certified course) one week before my senior season of cross country started and ran 18'05 with the first mile in the race run in 5'28 (17'05 pace) which at the time was the fastest mile I had run in my life. To say I went out a bit hard was an understatement! I obviously faded quite a bit and was disappointed with myself and "gave up" the last 300m of the race or so, which led me to miss out on a sub 18 clocking.
If I had paced myself sensibly I know I could have run 17'30 - 17'40. I only ran 3 races during my senior season due to me coming down with IT-Band syndrome (an injury that would plague me for several years). Of those 3 races before I was injured I finished 8th in the first, 1st in the second (my first ever win!), and 20th or so in the third (a much, much larger field).
I didn't run at all for 2 months. The next month I spent running 1-3 miles every other day. The next month I started doing easy runs of 20-40 minutes. Needing to do something athletic as per my school rules, I decided to do an indoor track race unattached. I ran an indoor 1500m race. Keep in mind this is after the injury and not doing anything but short easy runs for a month.
The race was on a 160m track and I had no idea what my fitness was let alone splits on a 160m track. I just went out with the group and "followed the train". I was the caboose and slowly became detached from the group (everyone else) in front of me. I ended up finishing dead last, much like my first cross country race. Except this time, it being a track race, I was also lapped by 3 people. I finished in 4'55.2 which is the equivalent to a 5'18 mile. All things considered it wasn't bad but I was unhappy about being lapped and wrote in my training log "never running a 1500 again".
I then started running up to 117 miles per week (7am, 10pm, 15 on sunday) ALL on a treadmill. I lived in New England and the roads weren't too great during the winter. All of my mileage at this time was at 6'45 pace. On the long run I ran it progression style finishing at 6'15 pace. I injured my hamstring from all the hard running (no surprise) and never got to do anything with the fitness. Was out for 4 weeks.
I never ran another race but I did do one workout later that spring that indicated I could have probably at least managed 18'15 - 18'30 in a 5k race (5 x 1600 in 5'47, 3 min rest). After that I came down with IT band Syndrome again and was out for the summer before college. I then had many small niggles during the fall. I then came down with IT Band Syndrome again in the winter. For the next year I had niggling injuries that kept me from running.
Today (junior in college) I am running regularly with no problems. The training is much more sensible as I actually know how to train properly now. Several months of training with regular workouts & no problems!
TL;DR - Even if you are the slowest person on your team when you start out, with hard work you can achieve things you and others may never have thought possible. Also, be patient and look to the long term. Good luck!