In South King County there are two major trails which intertwine and create many interesting loops and connections. If you only want to ride on trails, there are plenty of ways to go about that. (Kent has some useful maps here) My route also includes on road riding, especially during the southeast portion of the route. You can follow the river using Green River Rd SE and navigate through Auburn to get to the southern terminus of the Interurban Trail in order to form a figure 8.
Getting started
I had trouble coming up with a starting point for this one. I used to have Kent/James St Park & Ride, as the King County Metro 150 stops there, but if you are coming from Seattle, it makes more sense to get off at Interurban Ave S and 52nd Ave S. I eventually settled on Cecil Moses Memorial Park as this allows you to ride the whole length of the Green River Trail, and still has ok transit access with King County Metro 124 stopping nearby. You can also take advantage of Sound Transit’s 1 Line and stop at Tukwila International Blvd Station. It isn’t the most pleasant ride from there, but you can take Southcenter Blvd to the Green River Trail, or use King County Metro’s RapidRide F Line. If you are arriving by bike, you can take advantage of the Duwamish Trail and the on street connections through South Park. Once you’ve made it, now the easy part begins! Enjoy the winding riverside trail as you make your way south. After riding through the park, you make the first of many bridge crossings and ride through a former Boeing facility. Be careful after the next bridge crossing, as you loop around under the bridge, and it gets narrow here. After following the river a bit, the trail runs parallel to Interurban Ave S, making a good connection with King County Metro 150. Once you rejoin the river, you travel through Starfire Sports. There is a bridge with a narrow sidewalk here. You can either use the travel lane to cross, or use the sidewalk. If there are people walking, you will have to get off your bike and walk as it is a very narrow path. Just south of that is where the Green River and Interurban Trails intersect for the first time. Stay right to keep on the Green River Trail. From here it is pretty straight forward, just follow the river and make the occasional bridge crossing.
Recent updates
Once you get to S 212th St, you’ll need to cross over the river again due to construction around Van Doren’s Landing Park. According to the website, the construction is set to be complete Spring 2023, but currently it is still fenced off. After it reopens, it looks like this will once again be the preferred route. In the mean time, you can use the west side of the river which is mostly car free. There are many diverters along the way, and the roads that allow cars are mostly rural residential. The official detour has you going back over a trail bridge which also connects with the Puget Power Trail, but I prefer to go further south. I cross at the trail bridge which connects the golf course just north of W Meeker St. The bridge is wide, but watch for golf carts here. The nice thing about this section is the routes on the west and east side of the river are both good and well used. If you ride this route multiple times, switch it up and see which you like the best!
The road through Auburn
Soon after you cross the river, you will hit the intersection with the Interurban Trail. If you want to stick to trails, you can turn either left or right. The Interurban Trail is straight and uninterrupted for about 15 miles. This is the end of the continuous section of the Green River Trail. To continue on the road, we’ll use S 259th St. There is a short portion of the trail, but many riders choose to continue on the road which you can do if sticking to the river isn’t your highest priority. From there you start heading south as the road turns into Green River Rd. You soon hit the 277th St Corridor Trail. If you turn right, you can get back to the Interurban Trail. If you turn left, there is a climbing trail which will take you towards Lake Meridian and the Soos Creek Trail. Continuing south, Issac Evans Park is a good rest stop with bathrooms. A bit further south, and you will make your way to downtown Auburn. The streets can be busy here and you have a couple of options. If you don’t feel the need to do the entire Interurban Trail, and are ok with skipping the White River Trail, you can turn onto E Main St from R St NE and join the Interurban Trail from there. (Short route link) If you want to follow my route, I turn onto the busy M St SE from 3rd St SE. At this intersection you can wait for a break in the traffic, and a bike lane appears shortly after you turn. This bike lane does turn into a wide sidewalk, which I recommend you take, unless you are biking with a group and double up in the lane. Near the crossing of Auburn Way S, the sidewalk becomes narrow, you can merge into the road here. As most cars are turning left, you shouldn’t have much of an issue in the right lane. You then navigate to R St SE. Right after the crossing of the White River, turn right onto the White River Trail. This trail takes you through Roegner Park, another good place with bathrooms for a break. Once you are finished with the trail, I recommend sticking to the sidewalk on A St SE. The two left turns around Ellingson Rd aren’t the most fun, but after that the roads have little traffic. Navigate through Pacific to get to the southern terminus of the Interurban Trail. After County Line Rd SE (Yes, we are dipping into Pierce County!) there is a sidewalk trail on Valentine Ave SE. After you start heading north, it is smooth sailing from here, just 15 miles of straight trail.
Future updates
At the current northern terminus of the Green River Trail, the plan is to extend the trail north into Seattle city limits along West Marginal Pl S. This will get you close to the Duwamish Trail and the future Georgetown to South Park Trail, although 14th Ave S will remain a gap. Further south, a section of the Green River Trail near Starfire Sports is part of the new Lake to Sound Trail. This trail is being built in segments and currently has a couple of major gaps. Segment F will create a better connection alongside Southcenter Blvd towards the Tukwila International Blvd light rail station and SeaTac. Segments D and E will provide a connection into Renton continuing from the completed Segment A. This will be a useful connection to the Cedar River Trail and the Lake Washington Loop.
There are some very long term plans to extend the Interurban Trail further south. No date estimates or project pages, so it’ll probably take a while. There are currently two sections in Pierce County, one in Edgewood, and the other in Milton. Currently it isn’t very pleasant to connect to them, but it can be done. In the nearer future, there will be a trail in Fife that connects to Milton’s Interurban Trail. This will be able to take you to Tacoma and Puyallup.
Connections
I’ve mentioned many current and future connections already. On the north end, there is the Duwamish Trail. In the middle, there is the 277th St Corridor Trail and the Puget Power Trail. One I haven’t mentioned is at the southern end of the Interurban Trail. Stewart Rd SE gets you close to the Sumner Link Trail. The idea of this trail is to provide a link between the Interurban Trail and the Foothills Trail, but falls a bit short on both sides. Along with trails, there are also connections to great rural routes. My favorite being continuing south on R St SE in Auburn which turns into SE Auburn-Black Diamond Rd. You can then turn right onto SE Green Valley Rd. (This is part of my Foothills Trail Loop route)
Seattle Bike Routes 
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