Hey thanks! IMO not much of a difference in terms of training approach. I think plenty of guys who break 2:40 can progress down to sub 2:30 over time and if they stay healthy and can run consistent high mileage. Now going from 2:30 to sub 2:20 gets a bit harder!
Basically the faster you are, the closer your half marathon pace gets to Lactate Threshold Pace (60-min race pace).
The whole goal is usually got get your marathon time to be within 2 x (half marathon PR + 2-3min). This "crunches" down the whole pace spectrum so that Marathon pace is marginally closer to Lactate Threshold pace.
Of course it depends on the genetics of the individual runner, their training history/volume and mix of appropriate speed workouts over time. There is a certain periodization to maybe working on 5km/10km and xc speed for months and then transitioning back and forth to marathon training/racing blocks. However, the basic marathon training principles are pretty similar...the spectrum just shifts to "8-12 seconds per mile faster than goal pace" for a lot of workouts....some maybe 15-20 sec/mile faster and some Long Run efforts maybe even 10-20sec/mile slower. So working out in those areas: Aerobic Threshold, Lactate Threshold/CV. Breaking 2:30 is usually a very serious endeavor (usually only very dedicated runners can pull off) that for sure takes some natural talent and probably consistently high mileage for several years.
Good luck!