HI Laura
Personally I'd say yes, you can do it - but then i'd ignore the RW training plan anyway...
TT is very flat, so it's much more of a running course (which is the one thing i'm concerned about, I like hilly ultras with built in walking breaks and nice runs back down!) which will probably suit you down to the ground.
I would say have a look at a few online training plans (http://www.trailrunevents.com/ul/schedule-50k.asp is one of the more favoured ones by a few people on here) but then remember that a schedule is only a guide. To be honest I did a 30m and a 60m event this year on less training than I should have, a lot of ultra running is mental strength, and if you're a multiple ironman I'd think you'd have a fair bit of that!
Also, I'd say try to do a lot of your running off road if you can - it will build strength up, both muscular and joints, and is a damn sight more fun than slogging along a rock hard stretch of poxy tarmac (can you guess i'm a trail runner?!)
Cross training on a bike will certainly help - but I wouldn't replace a long run with it - you need the long runs more than anything to develop the sensation of running for that long, of what it's like to be on your feet for that period of time (and to get used to carrying your kit etc, making sure you're trainers fit etc) - there's a big difference between being on your feet for 5 hours and being on them for 10...
Just a quick answer - for more info or advice just ask - or head over to the Wannabe thread on this page, although most of us on there are now ultrarunners it's still full of advice (and lots of madness LOL)