Sorry to butt in, but i recently did Thames Meander (55m with 4kg pack, & getting lost!) in 11 hrs (3rd lady).
Basically as mara training (my mara times average 4hrs) with some longer long runs every other week - 4*30+, 1*35+. biking & IM might be an advantage as you'll be used to more time on feet? Not saying my training is the optimal but it worked!
Which 50m are you doing? To be honest i wouldn't worry about it too much, especially if you've done IM - there's nothing magic about 26.2, & in some ways i found ultras easier 'cos you get to walk the odd bit, eat malt loaf, pace yourself, etc. - definitely friendlier than an 'as fast as possible all the way round' mara.