First of all, I would like to say thank you for the all the effort you put into this website and your YouTube channel. They have been a big inspiration and aid to me with my running carreer so far.
However I am experiencing something that I cannot find anywhere online that has happened before. I think i may be a naturally slow runner, I train relatively hard at swimming most of the week and try to get in some runs whenever I can but the problem is I am well below some of other guys in my team who i feel I am much more fitter than.
I currently run a 23min 5km and im aiming to get that down to sub 20 by the start of the cross country season (May). I think one of the causes of my lacking ability to gain on my time may be because I am quite a muscular teenager because before I found running I was a gym nut and put on a fair bit of mass which I now regret a lot. Any advice for my situation would make my day, thanks for your time
First of all, thank you so much for the kind words! Now, hopefully I can help you. You have a good problem actually. What you have is a lot of endurance, but a lack of speed. There are ways to improve that. The first thing you need to realize, is to get faster, you have to add speed to your workouts. Doing longer slow distance runs will help you for your overall endurance, but in actuality, if you don't have the speed, you won't be able to run a faster time during the cross country season.
I suggest when you do get the chance to run, to work in some strides, or pick up the pace during a run. I call it "touching speed." Hill runs, fartleks, doing some sprints after your distance run is done, or intervals are all different workouts that can make you more comfortable with a faster pace.
As for being more muscular, that is something that you can't do much about other than not lifting that much anymore. Being a swimmer probably affects that as well, since you are using your upper body to pull your way through the water. If you stop lifting, you'll naturally lose some of the muscle mass, but it won't all go away. Just enjoy the fact that you probably look better at the beach than a typical skinny runner.
Here's are two videos that might help. The first is about running stamina, which means holding a faster pace for a long time like a cross country race. The second is on a workout called Fartleks. It's a funny word, but a serious workout.