In reply to tony:
I'm 38. My normal steady pace without pushing or specifically training is around 4:50/km for training runs up to half marathon, whilst my fast pace would be a around 4:20, so I was either planning on aiming for 3 hours 30 (4:58/km) or 3 hours 15 (4:37/km).
My ultra pace for the sections I can run I would expect to be much slower, around 5:30/km, so I'm just a bit concerned that the gap in pace is too big, and that I kill my legs for the ultra if I run my fast pace during the marathon. I presume that should only do a few short runs at ultra pace, in between the two events??