Willeton to Mundaring townsite? That is quite a return trip no real direct route but one I would suggest:
From Willeton short trip across to Willeri /Roe Highway intersection to pick up Roe Hwy Cyclepath & ride to end at Welshpool Rd.
Right onto Welshpool Rd for few hundred metres,
Left onto Hale/Hawtin Rd to Kalamunda Rd
Straight over to Gooseberry Hill Rd
Left onto Watsonia Rd.
Right onto Ridge Hill Rd Rd (climbing starts)
Past the scenic Zig Zag ride on your right at top of climb continue on fast descent to a tricky roundabout & left onto Helena Valley Rd.
Immediately turn right onto Scott St.
NOW you have a choice you can continue up Scott St until it meets the Great Eastern Hwy , then ride directly to Mundaring townsite about 10km? along Great Eastern Hwy, about 15 years ago I would ride strength-endurance up that climb but nowadays the volume of traffic plus the interstate-bound trucks I wouldn't touch the climb.
Instead I would cut through Darlington & Glen Forrest on roads that run alonside the Highway there are a few short sharp climbs but no traffic & the roads are used frequently by cyclists on the Mundaring-Kalamunda-Zag Zag loop of around 50km.
One word of warning on the Roe Highway cyclepath the suburbs of Langford /Beckenham (Nicholson-Welshpool) are a touch 'edgey' just be alert riding through them after curfew
Any more queries you can PM or drop into the shop.