Rather than long johns, which can be a hassle when you get back indoors, consider some mesh or textile overpants- Since your weather is moderate- the mesh may be more than enough to take the chill off-
I have the Olympia mesh, and because they have armor in the knees they keep you warm- i'm ridden them in 25 degree weather and did fine...30 seconds to put on or remove, so great for commuting.
1980 Vespa P200E
2006 BMW R1200GS
2012 SYM HD200 Evo