I definitely found my 30mile race easier on the legs than any of the marathons I have done as I had no time expectations really so was happy to walk when needed and there is generally much less tarmac on an ultra, so it is much better on ur joints. Think it is entirely mental too, so u might have only ever run 20miles in training or a marathon but u know u get to a point where u can just keep going. That then transfers into life- u know u can just keep on going. Long distance running and childbirth are probably the two most empowering things u can do. Both give u a huge amount of confidence in what ur body can achieve and leave u too bl**dy knackered to give a sh!t about the unimportant stuff in life ;0)
There are far more experienced than me but my advice would be to follow a marathon training plan. Think general advice for most ultras is that don't need training runs over 20-22 miles. For a get u round at a easy pace I would go with that. If u had more time before race and had done an ultra before and wanted to go for a faster race then maybe back to back runs would b a good idea. But for 1st attempt and with no experience of back to backs I wouldn't. Lots of runners on the ultra running FB pages don't do back to backs even for much longer races. Some people just break if they do back to backs too.