From downtown you can hop on the Burke Gillman Trail (Head north from Sheraton on towards Gasworks Park to meet up with the trail, I suggest going up Westlake rather than Eastlake Ave). Once you hit the “Burke” you can follow that towards the Sammamish River trail and keep following that. The route is pretty much flat and mostly mixed use path, but it is probably the best bet… If you follow it all the way around and down East Lake Sammamish Parkway (no longer bike trail) you will run into the I-90 trail. From there you can head back West and either cross the bridges, or head down south around Lake Washington.
When are you looking to do it? Weather has kind of sucked recently so I’m not sure it will be the best riding until Spring (or pack your cold/wet weather gear and bring a couple of spare tubes). I might be up for joining you if the weather isn’t totally pissing outside as I haven’t been on my bike in 5-6 weeks and it would be good to get one last ride in before I commit to the trainer for the next 6 months.