I've used light rail between Redmond and Seattle a couple times. It's "basically" great!
But - the early start and late finish of a 400km may not mesh (based on Mick's note, they don't), and they're still a bit in "shakedown" phase.
I think yesterday passengers were being shuttled between two of the Bellevue stops by bus (they were talking about it on Saturday evening, last time I used it). Not sure if they even had plans to accommodate bicycles with that detour.
Wonderful as using it has been, i'm not sure I'd want to be dependent on it to get to the start of a 400km before 5 am.
If you're staying in the U District, and in really good shape, you could ride to the start. (I rode Woodinville to the UW and then one day Seattle to Portland - once - you don't have to a total superstar to do it.
ronald