If you can get to Marymoor Park, you really don't need to worry about GPX files. Once in Marymoor, just follow the paved trail and it will take you to the Sammamish River Trail. When you get to Blythe Park in Bothell, watch the signs for the Burke Gilman trail (the trail has a few branches, if you end up on a road you have gone the wrong way, just go back and take the other branch). Stay on the Burke all the way to the UW or Gas Works - I assume that the detour in Lake Forest Park is well marked, but I have never ridden it so I cannot say for sure.