Jeremy: fair points.
Carterusm, I'd suggest looking for something in late July to October, probably in the 30 - 40 mile range, on the list mentioned earlier by Jeremy. Depending on how M'cr goes, you may feel wiped out for a few weeks afterwards (I did!), and then you need time to build up again, do some back-to-back long runs etc. I did the Croydon 30 (early July) 'cos it starts from my clubhouse, and then NDW50 (North Downs Way 50 miles) in early August - but I'd done the training for the London Ultra 50 K before M'cr, so I had a 50K and a 30M (and a number of training runs over marathon distance, with semi-long runs the next day) before I reached NDW50.
Royal Parks ultra might be a nice one to start with (easy navigation, easy terrain) but you need to decide and enter NOW before they run out of places where you don't have to raise hundreds of pounds for charity, so might be more practical (and cheaper) to wait and find another one.
Also, not all ultras are well marked - some you need to do some navigating on the way, and you want to check the terrain and course profile (London Ultra and Croydon Ultra some hills but not bad. NDW fair amount of ascent and descent; Lakeland 50 LOTS of ascent and descent).
Also, if you can find one that's fairly easy for you to reach and run in sections for training runs, that can be a great mental help on the day - removes navigation worries, and you know what's coming next!