Eventually the idea is to tie the foothills trail in with the Interurban via the river walk in Puyallup. What I understand to be an interim route connecting the two (southbound) is as follows:
0.0 Starting at Interurban trailhead in Pacific, turn left onto 3rd Avenue (freshly painted bike lane).
0.7 Turn right on Butte Avenue SE (The next mile is on a shoulderless road. It starts out in a residential district, but there can be truck traffic toward the southern end.)
1.7 Turn left onto 8th Street. (The first block is shoulderless and can have truck traffic. Immediately crosses a narrow bridge after which a wide sidewalk/path starts.)
2.0 Turn right from sidewalk/path onto asphalt trail. This follows the winding White River south through farmland to some industrial parks in Sumner.
Mount Rainier can be seen behind industrial park on Sumner Link Trail
Mount Rainier can be seen behind industrial park on Sumner Link Trail
5.2 Follow right branch of trail beneath Tacoma Avenue bridge.
5.6 Continue straight onto bike lane or adjacent sidewalk on Fryar Avenue.
6.2 Cross Bridge Street / Main Street and take second right. A 12-foot wide sidewalk serves as a path in front of houses on East Main Street then becomes a trail.
6.8 Bear left in park at confluence of Puyallup and White rivers.
7.2 Turn right onto path adjacent to East Main Avenue, cross bridge, then make a sharp right onto access trail to Riverwalk Trail. Turn right onto Riverwalk Trail and proceed beneath bridges to concrete barriers.
7.3 Go around concrete barriers, turn right up a short gravel grade, and look for a path through the bushes that goes around a gate. Continue south onto 134th Avenue (no sign).
8.1 Cross 20th Street and arrive at Foothills Trail. Turn left to reach restrooms and water.
(This queue sheet is from BikingBis) My earlier attempts at riding between the two usually involved Valentine to Milwaukee and I got turned around a couple of times between the end of the foothills trail and the intersection of bridge and main in Sumner.