Seems a big jump to expect to go from 3.13 to sub 3 unless you have massively increased your training and have lost a load of weight to help too.
your half suggests a faster marathon than you did but you probably need more long runs to get to your marathon potential. Some people are lucky but mostly you need to have done a lot of marathons and therefore long runs over a period of years to get the times suggested by the calculators
you say that 7.15 for 24 miles felt comfortable. Different people define comfortably in different ways. I have a friend who has just started running I tell him his runs mostly need to be easy pace, he says he is doing 8mm and that is easy but I don't believe it can be when his 5km is 2-3 min slower than mine and 8mm is not easy for me unless I am really well tapered.
If you were in sub 3 shape then MP would be around 6.50 so your long run was far too close to MP. I do my long runs a minute or more slower. If you found it what you define as comfortable then another 2 miles at that pace or a bit faster would have got you a PB.
The he fact that you struggled to do 1.30 to halfway when you did a 1.24 half suggests that either you set off far too fast (you don't give us your 5km splits) maybe you set off at 1.20 half pace and slowed?? Or if you were running fairly consistently to halfway yet struggled then maybe just not your day or you overdid that last long run and were not recovered.
in my earlier days I did ashby 20 5 weeks before London, ran it quite hard but not quite raced it. Started the same pace at London When I was tapered and when it was flat and could only hold it to 17 miles which is odd as I was tapered and it was an easier course. I think I left my race in training but also I think the weaving at london can use a lot of energy
I've done 24 hours and lots of ultras in the Gorun ride and Gorun ride 2. Have just got the gorun3, not done much in them yet but to me they seem very similar to th. Gorunride2 but maybe a little narrower.
i also have the ultra, I did 40 miles in them last sat no problem. They have a lot more cushioning but not a lot of extra weight. I prefer the rides to the ultras but only because I prefer lighter and don't want all the extra bulk but if you are not used to lightweight shoes then the ultra a good starting point
I've done this twice, great race, even I didn't get lost it is v well marked and loads of runners so you are never alone. A couple of times I went the wrong way but because the glow sticks were every 20m or so I soon knew I was wrong
first time I used alpkit second time a much better one and it Made a big diff
i remembed loads of food at CP there was def cheese as a friend I was with loved that I liked the cake it was a bit like madera cake, also remember there was coke and cereal bars