From 17th, make a left on Umatilla, take it all the way to
the Springwater-on-Willamette path. Turn right on the
path, take it to the Hawthorne bridge. Go up the eastbound
ramp, which leaves you on Hawthorne.
Not sure if you can ride Hawthorne all the way to 45th, but
you should be able to find a parallel street, like Harrison, if not.