What happened at around 20 Miles? I ask because I only have 7 weeks left to my first and want to try and identify what I am facing
I think that unless you are up in the top flite then you can only really compare against your own performance. For Parkruns etc. I just go round at whatever feels comfortable at the time. I used to be concerned if I was slower but I am starting to be less bothered by it now
It is very common for marathon runners to start off too fast. They think the pace is sustainable as it still feels easy. They might have even done some shorter races that would perhaps indicate they could run a certain goal time (there are table that can calculate marathon time form a 5k/10k etc) without realizing that the predictions are only accurate when suitable trained (e.g, typically they assume 70miles per week for a marathon). More often than not somewhere around miles 18 to 22 they hit the wall. Despite increasing exertion, paces fall precipitously, often to a walk of shame. Attempts to get running again fail quickly. They might be on or faster than goal pace through the first 20 miles, and then loose an hour or more in the last 10k, crossing the line a miserable wreck.
The cause is simply they went too fast for their fitness. Going out even a few seconds a mile to fast is bound to lead to disaster, unless you happened to have a soft goal time. Now there can be some other genuine reasons to have a bad time at the end, often related to injuries, cramps, digestion/nausea, heat. But going too fast is the most common.
Pacing is critical in any race but especially a marathon. In a shorter race if you go too fast then you will sl;ow modestly at the end, you wont get the fastest time but wont have a complete breakdown. In a marathon a small pace error can lead to a DNF, or literally crawling on your hands and knees in extreme cases. You only have a finite supply of glycogen, around 2000 calories which is about 16-20 miles worth at marathon pace. You will burn some fat, but not quite enough. Gels and sports drink can top up a few hundred more which is going to be just sufficient to get you over the line, and no more. At the end of a marathon if you are racing hard, your blood sugar is so low that your brain starts to go all fuzzy and your vision ends up a small blurry hole in the center. One marathon at mile 25 i nearly hit the wall, apparently I saw my wife and kids and waved to them but have absolutely no memory. At the finish i asked my wife why she wasn't where we agreed to cheer me on! She has the photo to prove I was waving at her, but my brain was too glucose and oxygen deprived at that point. I got a good finish time, 12 minute PR
For a beginner start 30-60s a mile slower than goal pace, every mile cut 5-7 seconds until you hit goal pace. At goal pace, you need to experiment a little and find that sweet spot, might be that you need to back-off 5-10secs from your goal to keep effort and HR in the right place. As you get faster and more experienced then the same idea applies. I start about 20-30seconds a mile slower, and try and hit goal pace by mile 4. Once you hit mile 20 you need to see how you feel but can start increasing speed to just faster than goal pace
With fresh legs form taper, race day excitement, adrenaline, and people rushing buy it is all too easy to go way too fast at the start. Best to keep looking at the watch and get a pace slower than goal to begin with. Going faster than goal pace has an exponential increase in energy cost, especially before your muscles are all warmed up.
