General rule: (check your state vehicular regulations for more specific laws and restrictions)

1) You may ride a bicycle on any road that is not and interstate or limited access highway.
2) You may ride a bicycle on any road on which bikes are not expressly prohibited.

Personally, I am not intimidated by motor traffic, know how to ride with motor traffic so I generally just leave my house and go for a ride as apposed to driving my bike out of town to ride.

As for beginner's length of rides, that is rather subjective and dependent on terrain and intensity. Cycling uses different muscle groups than running but many former runners have become good cyclists. I would start out easy and work up. 70 miles per week running is a lot of running.....