My first post to t-nation in a very long time, fun. 8|
Lots of good advice given already.
One thing people never mention is "using races as a training tool". If you want to run a sub-5 minute mile, you'll want to enter some 5k races and try to keep up with the fastest people for as long as possible. Local 5k's usually have someone in the ~5-5:30 min/mi pace (definitely sub-6) for the first mile. You can do some research first to see "how fast the race normally is", it usually holds up. You might be surprised how much more power you have in a race setting, so this is a great tool to help you progress faster. Just don't do this until you've started incorporating speed work at those paces though. You'll want to be able to at least run 400's at low to mid 5 min/mi before attempting to keep up with those people. One thing you'll notice about them when they are running, is how relaxed they are. Focusing on relaxation is not addressed enough. In all of your speed work, whenever you push the pace, you should not be doing so with the "complete the rep!@@!" mindset. You need to always stay relaxed; tightening up to produce extra force leads to a quick death.
I used this technique several times and PR'd my mile several times as well - in the first mile of the race. I'm obviously pretty dead trying to keep up with people significantly faster than me, so you just end up hanging on for dear life to finish the race. What's funny is, you'll end up PR'n your 5k too usually. As an example, one of my races I PR'd my mile at 5:15 and then ran 2.1 more deathly miles at 6:05 and 6:20.
You should also enter 1-mile races if you find any of those - they are usually held at the end of the year. Those races are usually extremely fast, it's still pretty fun trying to keep up with those guys.
The most fun race I did was a recent 1-miler where I just decided to go full throttle from the start. Ran 4:10 min/mi pace for the first 400m, then 4:40 min/mi pace by 800m, 4:54 min/mi by 1km, and 5:20 min/mi by 1 mile. After ~800m, my stride frequency was still good but my legs couldn't produce force, it was a fun experience. lolz.
Doing "ballsy/aggressive stuff" and racing more will help you improve considerably faster than simply training by yourself.
FWIW, as of today my watch timed mile is 5:06 and official race time is 5:12.. So i'm not sub-5 yet, but close.
As for training, at least one long run a week is needed to maintain/build your base. But most people don't do enough speed. For most people, 2x/week speed sessions & 1x/week long run is probably a good start. Eventually some really light running on your off days could be good too, at least as a warmup before you lift etc. Getting to the track is very important. Repeats of 200m, 400m, 800m, 1km with short jog rest are very important. There's so many protocols, others have already mentioned some effective ones. The most important thing though is that you start to think of speed as a skill. Speed work improves running economy more than longer slower running, and it's going to take tons of running economy improvements to get sub-5.
Finally, just don't overdo it. Progress gradually & safely. Injuries can happen very easily in the running game, it seems.