That's pretty much the idea behind 800-meter repeats. Following Bart Yasso's formula, I'm planning to run all of my 800's from now until 5/20 in 2:50 or better. Does this mean I'll run a 2:50:00 marathon? Probably not, but knowing that I can do the workouts a 2:50:00 marathoner could do will help with my confidence, which can be a huge factor on race day.
For 5AM, I was feeling great. I ate a banana on my drive over and drank a few ounces of water prior to my mile warmup. I didn't save my 800 splits but my first was 2:47 and my last was 2:45. The three in between were in the same ballpark. I was shooting for 34-second laps and was hitting 33's mostly. I saw a few 34's but no 35's. I began to tire a little near the end but could have done more. Strength trained for 30-minutes afterwards.