Prosser Dam Road is in great shape right now for walking, running and riding.
Once you get to the end of the paved portion of Prosser Dam Road, via whatever means you choose, proceed along the dirt fire road toward the dam.
The first tenth(ish) of a mile or so is a bit rutted out but still easily navigable whether running or riding a bike. Riding a gravel or cross bike out there right now provides for a great workout while avoiding car traffic. Running out there provides for a nice, consistent, mostly flat route.
Once you cross the dam you can veer right and head straight uphill along Lloyds Loop. It’s clear of snow and mud but it’s not marked and not easily followed.
Alternatively, you can continue north on the fire road for another 2 miles where you’ll reach your first intersection. From here you can fork left and head toward Dog Valley Road or fork right and head toward Boca Reservoir, on Boca Road. Both options provide clear courses for quite a ways.
Both Dog Valley Road and Boca Road have various offshoots that provide different routes. Some lead to Hobart Mills, while some lead to Boca and Stampede Reservoirs. Some routes become a bit muddy and even icy in colder temps as you near Boca, but still very much navigable. Run or ride to your heart’s content!