My view is that if you can do a 50 with about becoming an absolute physical wreck, then you can probably do a 100 - physically
However the real challenge is mental
Take TP100 for an example.
start at 10.00 am, run all day until day light fades at say 7.00pm. You will probably have done about 50 miles in 9-10 hrs (ie you'll have done a 50 miler)
you now have to do it again, except that
1) you are already knackered
2) it will be dark for the next 12 hrs you will be racing for
3) you could quite easily be alone for 8-10 hrs
4) come midnight you will be sleep deprived and it will stay like that for another 8 hrs
5) it will probably be sub zero and possibly raining
6) every check point will offer you the chance to sit down and rest, and stop
7) you will know that people have already finished and you still have another 35 miles to go
8) you will be moving slower than you ever have in training and know that you need to keep that up for another 6-8 hrs
9) your feet and legs will hurt like never before and you know that you will have to keep it up for another 6-8 hrs
10) you will convince yourself that it is not possible for you to finish
11) you will almost fall asleep whilst standing upright
12) you will be so tired that you don't bother to eat or drink at aid stations
13) you will know that there is no shame in not finishing
14) you will start to rationalise why you didn't finish in a away that you can explain to your friends and family
IF you get through these issues, you will suddenly find that you have been running/walking miles without remembering it. Your mind will disassociate itself from your physical side and you will overcome and you will finish
BUT it is very very difficult
This is why 100s are as much about supporting others as they are about supporting yourself