Biggest change I made was changing the pace I was conducting my long runs at.
My PR for the marathon prior to breaking 2:20 was 2:40:02. The marathon is not like the 5K or half-marathon. There are a lot of GREAT runners who never get the marathon completely perfect. Be patient, you are only 23 years old. The biggest advantage you have right now is time. I started running marathons a few months after joining the Army at age 25. I ran 2:19 at age 31 so it took me 6 years to break the 2:20 barrier. There are runners like you who very well may do it in your early to mid 20s. How long does it take? It can take months or it can take years. Depends on a lot of factors.
I will say if you want to run under 5:20 per mile for 26.2 miles your long run should not be long and slow every weekend. Mix it up with some faster segments. I would do 20-24 milers at altitude with very hard miles thrown in around 4:50 to 4:55 then back down again for 8 to 12 miles at 5:35-50 pace. That being said, the following one or two weeks were much easier pace. Recovery is just as important as the hard sessions and that is where most runners (including myself) get it wrong in training. The hardest workout I did and do is my long run. Just my two cents. Folks here will bash you no matter what you do. Break 2:20? You got lucky. Course was fast. You're nothing unless you break 2:05. Bottom line, did they do it? Sub 2:20 is very difficult to break. Sub-2:30 takes an enormous amount of fitness and grit to achieve.
"Players don’t take feedback or criticism from spectators. Spectators rarely have success in their own lives because they are too busy spectating on the lives of the successful. I value correction and coaching from other players, but will never value criticism from naysayers or spectators." - Grant Cardone
I listen to professionals who are better than me and will offer help to anyone who has the grit and drive that it will most certainly take to break 2:20:00. You have the chops. At 23, asking this type of question already tells me you have the capability. Keep working on your leg speed. Remember, the trick is getting lactic acid to clear faster than it is building up in the blood stream. This physiological adaptation does not occur from long, slow easy miles. There is a place for easy recovery miles but I know what it took to break the 2:20 barrier and it was a lot of high end running, at 6,000+ altitude. Mile reps in the 4:45-46 range, 2 mile reps under 10:00, long runs averaging 5:30-35 pace, hard tempos, long hill repetitions all out etc.
We're caught up with running high mileage and forgetting it is the quality, not the quantity of the miles run that counts. My highest weeks ever were 140 and 142 miles. Pointless. It was more of an experiment to see what worked and what didn't. High mileage was not the answer. I broke 2:20:00 off of 85-90 miles a week. Hope this helps and good luck to you.
Nate
www.rundreamachieve.com
nathan@rundreamachieve.com