At 21 you can do anything you want to do. This doesn't take much effort just perseverance.
From Hadd a few years back, "...then you need to learn how to run fast. This is a skill you need to develop especially if you are a heel-striker. You need to learn how to land further forward on your foot.
Running uphill helps to learn this because it becomes increasingly more difficult to heel-strike the steeper the hill. To get an idea, find a steep ~30 degree uphill with a hard tarmac surface (rather than soil/sand/gravel) and run up swiftly for 30-50m without letting your heel hit the ground. Compared to distance running, accentuate your knee lift (without exaggerating). Drive off the rear foot more like a light bound than a normal jog uphill.
Some (even good) distance runners can have real difficulty learning how to do this, so do persist and don’t give up because you “can’t get it”. Note that this is skill-learning (and not fitness training), so just run for 5-8 secs each rep and allow 60-90s recovery. You must be mentally/physically fresh when doing this, so don’t add it on to the end of your long run. Instead, do it BEFORE your aerobic run (but after 5-10 mins jog warm-up and some dynamic stretching).
Be cautious and do no damage to your lower-leg/Achilles (you won’t know for up to 2 days afterwards, so be cautious till you see the physical cost of the session). Don’t do too many reps, 6-10 x 30-50m is plenty for one session. Remember, it’s a skill, so doing it 1,000 times but badly, will not help. It’s a skill and some people get it quickly (1-3 sessions) and some take longer.
When you think you’ve got the hang of it, start running 10 x 100m on the flat with a minimum of 90 secs walk-back recovery using this newly learned mode of running. Any stretch of flat road will do; so no need for a track or to buy spikes. We’re not looking for a world-record here, but (aiming for 25:00/8k) you would expect to develop the skill to hit ~13 secs for 100m on demand (at least in the last 1-2 of the session). Whatever ever your 8k target, you want to be able to hit the kind of pace you would expect to run in a 300-400m race, at least for the last few reps of the session (although the earlier ones can be slower and build to quick efforts for the last 2 reps).
It’s okay to start each session with 2-3 reps on your 50m hill (if nearby) just to re-calibrate the skill, and even to run the first few reps as 60m rather than 100s.
At all times, remember, this is a skill. It has little to do with the fitness level that allows you to run a decent 10k. So doing 25 x 100m in 17s is not going to help you gain the ability to run 8 x 100m in 13s. Just stick to 8-10 x 100m and aim to improve the skill and run them faster as the weeks progress.
Learn how to move, as I have outlined above. And I am very aware it would be much easier if you saw what I was advising rather than just reading these words – I know most runners learn best by observing and copying, so posting some video clips might be an idea for the future.
Keep this skill session in every week. So now your week might look like this:
Tues: long endurance (as earlier)
Thurs: medium endurance (as earlier)
Sat: short and fast (as above)
Hope this helps."