An ultra is to be respected (as is someone who attempts to run one).
From my own experience, although I had run a few marathons, I found the step up to ultras to be quite tough and best done gradually - an important part of my training was to allow time to rest, recover and replenish - that didn't mean doing nothing - I would have maybe two weeks where I trained relatively hard (building up the mileage) and then a step back "easier" week.
I also found that the distance involved with an ultra is just as much a mental challenge as a physical one and required a change of mindset (after all, who in their right mind would run an ultra) - rather than tackle each long run as a 20 or a 23 miler I'd divide each long run into sections - a half marathon followed by a 10K or a 5k followed by a half marathon followed by a 10k.
I'd start off each long run at a shuffle - barely a jog - and build very gradually into a pace well within aerobic threshold (a pace I felt I could comfortably run at all day) - at first I didn't worry too much about how much distance I covered - I'd focus more on building up to running for two hours and then three hours and more - only when I thought I could run comfortably for long periods of time (sometimes walking on the steeper uphill sections) did I start to consider distance.
I found that getting used to eating and drinking on the run (small regular amounts and easily digestible) to be an absolute must.- I'd carry energy bars, and isotonic drinks with me or follow a route that took me past a petrol station or two or a village shop where I could stop off and get something.
Over time the body adapts - aerobic threshold increases as does lactate threshold (the point at which legs turn to lead) - after putting in the mileage (there's no other way) I began to recognise the right pace to run at - managing to just eke out gradually those energy reserves bit by precious bit.
It does take time and experience - once you get it right though it's a terrific feeling knowing that with 49k behind you and with only 1 k to go there's nothing (not hell nor high water or any any other type of obstacle) that's going to stop you getting to that finish line. Good luck.
When I was training for ultra marathons I'd run home from work from London E1 to Gravesend in Kent - some memorable hills included Vanburgh Hill in Greenwich, Shooters Hill between Kidbrooke and Welling, East Hill in Dartford and Stonebridge Hill in Northfleet.
Another good long hill I used to train on is Chiselhurst Hill.