I'll start off by saying that I have one run 1 sub 3 hour marathon, and that was in April of this year! I may not be an expert, but I have learned a little of the last few years.
I started running in summer 2013 and like lots of people, I decided on a 3 day a week running plan. The FIRST plan gets you to run one set of intervals, 1 tempo run and one longer run along with cross training on the other days. I was an unexperienced runner at the time, but I managed a decent half marathon in 1:39, but in the Istanbul marathon in November, I blew up big time and hobbled in for a 3:56.
In 2014 I started running more. I set myself a target of a sub 1:30 half marathon in March. I increased the number of runs a week and could get up to 50–60km per week in the race build up. I also started to add in slow runs, and I mean slow, like 6mins/km which is about a minute per km slower than I had been running. I also did some intervals once a week and an over-distance run.
I managed my sub 1:30, and even did a 1:27:30 2 months later, as well as a sub 40min 10k. I was feeling gooood!
I decided to increase my mileage late in the summer, to prepare for Frankfurt Marathon in October. I touched 100km in a week for the first time, but I started to fail in my interval sessions. After getting slower and slower for a while, I saw my doctor and found out I was anaemic.
This isn’t as rare as you would think, so I started on the iron tablets and started feeling gradually better. I couldn’t get back to 100% in time for Frankfurt, but I did manage a 3:19.
Over winter and spring I started doing more interval sessions and try to drop a few kg. I was training for Bonn marathon in April, by way of 2 half marathons and a few 10k races. Training went great again, I tried to do intervals twice a week, or once in race weeks. I was only running 50–60km per week, but getting faster and faster.
I got my 10km down to 37:22, my HM to 1:21:50, but the bad news, low mileage running isn’t great for marathon training, so I only managed 3:00:15 in Bonn!
Since last October I really stepped up my level of training, doing a Jack Daniels 10k plan, the going higher distance, with up to 125km per week. My training went really well and got me a 1:18:50 half marathon in the build up to my marathon, but unfortunately I didn't run well on marathon day, still managing a 2:55, but not the sub 2:50 I had been expecting.
So, how to run a sub 3 marathon? Unfortunately there is no big secret, you need to run consistently for a long period of time. You need to do intervals at or above threshold pace 1–2 times a week, you need to run long runs of 30km+ at least every other week in marathon training blocks. You need to race shorter distance races for practice and to find out where you are in your training, and you need to run slowly several times a week.
If you run 6 days a week, run 3–4 days easy, 1–2 days intervals/tempo, and once longer. Easy!