Unfortunately there's no easy way to the path from the capitol building.
I don't ride out there very often, but my guess is that it's probably
easiest to ride out toward 'Old Sacramento' first, which is a mile or
two west of the capitol. The bike path is north of the river so you
either have to cross on Highway 5 (a 65+ mph freeway, legal, but not
suggested), or just west of the freeway there's a bridge that is no
longer open to through traffic (just bikes and cars entering the park)
in Discovery Park. At the north end of this bridge is where the bike
path starts. Head eastbound on that path and you'll eventually (~30
miles) end up in Folsom. It's very flat, except for the last few miles
into Folsom where there are a few hundred feet of climbing. If you have
more specific questions feel free to e-mail me and I'll try to figure it
out.

-Rex