Did you sign up for your local Turkey Trot? It is that time of the year people tend to eat more, move less, and become a slug on the pavement. Strap on the NoBulls and let's walk through a simplified philosophy for you to improve not only your mile time but your cardio.
Five days a week, five different runs. Don't want to commit this much? I'll show you what to keep at the end for an adjustment.
Improving your cardio capacity and your mile time isn't about just running hard, it requires you to run at different paces and produce a different stimulus if you want to see results. Why? Simply put, if you can improve your max output(anaerobic) AND your ability to keep going(aerobic) both the ability to achieve an average higher pace on average and hold that pace improves.
Way Above Pace ( 85% < )
Just Above Pace( 75 - 85% )
Pace ( 75% )
Just Above Pace ( 75 - 65% )
Way Below Pace ( 65% > )
Assume you run a 10-minute pace for ease of digestion, pacing is your average mile time, not your best ever. Most people who run five days a week run beyond 1 mile in a session so this is more for those looking to achieve 3+ mile runs. You can do the % of a 10-minute pace yourself, but this is the layout. If you want to do just 3 days a week, runs 1 3, and 5 should be used as that will have the most variance.
How do you proceed? Two ways, either pick a certain amount of time to run for and use the pacing guide above for that run. Or understand your long-term mileage goals and as you lay out your weekly mileage total across the week you adjust your days according to the pacing required. The general rule would be higher pacing days are shorter runs and vice versa.
I am by no means an awesome runner, but I have seen results with this general layout. The more advanced you get obviously the more the details matter. Highly recommended combating this much running with weight training to support bones and ligaments.