The Cedar River Trail is a seventeen mile hike that is excellent to do from the late Spring well into mid-Fall before the first heavy rains. During the Fall and Winter when the Cedar River is swollen with rain water parts of the trail or underwater. Though, there are still sections of the trail that can be hiked. But if you want to do a half day or full day hike, if you walk slower, late Spring to mid-Fall the the best times to give the Cedar River Trail a try.
The Cedar River Trail runs from Landsburg to the mouth of the Cedar River in Renton at Lake Washington. Part of the trail follows what at one time was railroad tracks. This section of the trail has been converted into a wonderful hard packed gravel trail for about five miles. The remaining twelve miles of the trail are paved. This trail is also very popular with bicyclists, so it can be a bit busy at times. As with any busy urban trail just be sure to respect others and share the trail with others that are enjoying it.
If you do the entire hike or start from the mouth of the Cedar River at Lake Washington much of the first few miles of the hike will be trhough the city of Renton. This section can be a little confusing. If you follow the north side of the Cedar River for most of the way you shouldn't have too much trouble staying on the trail though. Once you leave Renton and begining following the Maple Valley Highway though, the trail is mostly in the woods from here until Landsburg.
Parking near Lake Washington is limited to four hours, so you may need to take this in to consideration when you are planning how long you want to hike. Are possibly have some one drop you off and pick you up if you decide to do the entire hike. The Washington Trail Association has an excellent article about this trail with good details on where you hike. It also has directions on where to start if you only want to hike certain sections of the trail.