Edit: Thanks everyone! It looks like my main worry (that typical ultra runners are just a lot tougher than me, which is still possible) has been answered, and there's things that I can work on. I've assembled all the wisdom here into a 100 mile bible for myself, as the basis for a plan to move forward with.
I’m really struggling with a fundamental aspect of pushing ultra distance. Its just me going solo and I have no peers around to consult, so bear with me.
On my four 90K-100K ultras (all with ~4500 feet of super gradual elevation), I always hit a point near the end where my legs go from tired and slow but tolerable, to extremely sore, within less than 10K. Sore enough that when I try to run, its so hard and slow that it’s no faster than walking, and feels like its wrecking my legs still further at a very high rate. Its a familiar feeling from when I first started running 5 years ago, when I went from 10K, to 21K, 30K, 42K, 60K, 75K. All those times I found that after I recovered, I could go then go further before my legs got totally wrecked. Now though, i’ve failed twice to go past 100K, and going 100 miles is feeling impossible. Its not getting any easier. I see people online that look outwardly no more fit than me, still periodically running after 200 miles, and I don’t get it at all. I'd like to be able to run at least a tiny bit near the end of a 100 miler, and not just deathmarch 20 miles.
What am I missing here? Do my legs just need more experience? I’ve done 4 x 90K-100K, and 4 x 50K.
Do I just not understand the level of pain that needs to be endured?
Other info:
I used running/walking intervals of 4 minutes/1 minute on my last 100K.
I’m old - mid 40s.
They are all solo & unsupported, and i’m carrying about 8lb on average.
I realize I consistently over-drink water, going to the bathroom every 30 minutes on average.
Been running about 5 years total, with consistent mileage all year round.
Last years’ mileage: 2000+miles/3250km
Typical off-season weekly mileage: 34 miles/55km
Strength training: In off season, heavy weekly leg strength training sessions. Very light maintenance sessions while in heavy running training blocks. Been doing it 2 years now.
Training blocks are always about 12 weeks, and peak weeks have now hit 120-130K for me.
Nutrition during ultras: I aim for 60g carbs per hour, with about a 50/50 mix between powdered drink mix and solid food like cookies, savoury stuff, etc.
I aim to replace about 600mg of sodium for every litre I drink.