Hey so short story for background to start out-
I ran my first cross country race when I was 10 and finished dead last lmao.
Ran my second race when I was 13 and finished 74th out of 75 people lol.. I was a good athlete but I just assumed I wasn't born to be a long distance runner so I quit then for good, or so I thought.
I started running again in 2011 when I was 17 casually to help motivate my overweight brother... I was probably running 5-10 mpw with no speed training or intervals, just running for fun. I played baseball/hockey/soccer/badminton so I was in good shape at this time but didn't take running seriously. I timed myself for fun in summer of 2011 and ran a 28 min 5k. Had no idea if this was good or bad. Decided that for my senior year (2011) I wanted to play every varsity sport so I ran cross country. I think my best time that year was 26:50 for the 5K in Fall 2011. I ran a lot that winter but only intervals and speed training because I was also training for baseball and playing hockey. Probably running 10-15 MPW. In the spring of 2012 I tried my hand at another 5K and ran 22:40 this time which got me pumped up a little bit because I was surprised at the improvement. I was attending a local college on a baseball scholarship but I also now had an interest in running cross country, but you needed so run Sub-22 mins in the tryouts to make the training team so I trained pretty hard in the summer doing a lot of speed work, probably running 15-20 MPW. In the fall of 2012 I ran a 21:45 tryout 5K on a hilly cross country course. Trained about 3-4 times a week with the cross country team at my college and by spring 2013 I ran a 20:25 5K on a road course. This was my best (and last) race as about a month after this I got signed to play baseball professionally and quit running to focus on that, obviously.
TL;DR- Improved my 5K time by 8 minutes in 1.75 years running 10-20 MPW.
Fast forward to now and my baseball career just ended (tommy john surgery) and I really want to start running again .. I'm starting a 2 year MA program this fall and I really want to run on the cross country/track team. However, I'm now 40lbs heavier than I was in 2012 and just started running again 2 weeks ago. To make the "training team" at the school I'm going to I need to run a sub-20 5K. I feel like this probably isn't possible for my first year, so I'm aiming for a good 5k time for Fall 2021 and Track season in spring 2022. However, I really want to run sub 18. That would put me in the top 7 of their team right now and would put me in the races. I'm ultra competitive and I really want to do this. I'm also committed to putting in more mileage than I ever have.. my highest being 20 MPW.
So my questions are- 1. Is it possible to run a sub 18 5K in 2 years of hard training?
2. I ran a 20:25 5K at 18 after 2 years (ish) of training. Based on what you read, is that my peak?
3. How important is high mileage to running good times?
Sorry for the newbie questions guys, I honestly just have no idea about a lot of this as most of my life has been focused on other sports.