As my earlier post. If you have a few months I reckon similar to the RW marathon training up to about 1 or 2 months before. Then a couple of 20's & then some 10m plus for 4 or 5 days on the trott. Don't worry about pace. That's built up in the marathon schedule. As for the long runs the slower the better, I found time on feet better than distance. e.g. RW 3:30 marathon pace training was 20miles / 3 hours. If you hit 20 in less than an hour than keep going until you hit the 3 hours or vice versa. Whichever is the latter
Did Ridgeway footpath ultra in November. 29m (everyone got lost & done at least 31m) /27m/26.2m Done Berlin Marathon just under 2 months before. Pretty much the RW training for that & followed it up by resting for a couple of weeks then getting a 20 miler in and 4 days running of very gentle 12-13 milers to get used to running on tired legs. Some people I know go for run/walk at certain times but I found walking the uphills my best strategy even if I was capable of running them, saves the legs for the next couple of days.
When I done my first 20miler I found 1min per mile slower than 1/2 marathon pace was the best. Wouldn't worry if your training runs are 15-18 miles. If you can do 18 and mentally think you can do 2 more miles then go with that pace