There are a number of informal trails in Upper Darby along Darby Creek. In the northwest corner of the township there is a nice walk along the creek (shown in orange in the Google Maps overlay above). The sewer authority keeps a wide path clear from State and Township Line roads (by the Hess station) through Pilgrim Park up into Haverford Township by the end of Burmont Road. Rather than walk through an area near the south end that can be muddy at times, bear west along the rear fence of the tennis club and then walk upstream along the bank of the creek from there.
South of there, another section of Darby Creek (shown in red) is accessible southward from Bloomfield and Rosemont avenues down an access road into the township's Addingham Tract and passing under the Garrett Road bridge.
A third section (shown in blue) can be accessed by walking along Creek Road west from Dennison Avenue and Blanchard Avenue, all the way upstream to the historic Swedish Cabin. There is also a path along the creek within Kent Park, a bit downstream off of Bridge Street. (When the county's trail improvement project connecting Kent Park to the Swedish Cabin begins, there may be blockages from time to time in these areas.)
Along the Naylor's Run corridor, you can explore Naylor's Run Park (shown in bright green), north of Garrett Road, which includes a section of the former Newtown Square Branch rail line. Southward from the same area, there is a walkable straight path (shown in light blue) that leads southward along the west side of Naylor's Run to Marshall Road and the Beverly Hills Recreation Area. Other stretches along the former rail line are also walkable on an informal basis.
Don't forget the lovely walks and runs possible on the roadways within the beautiful Arlington Cemetery (the light green area in the center of the township), which have long been popular with the neighbors. Please respect cemetery hours.
In nearby Lansdowne Borough you can walk through Hoffman Park and Shrigley Park, both along Scottdale Road south of Baltimore Avenue, as well as along some steep paths in Pennock Woods Park, off the end of Pennock Terrace.