General rule is no more than a 10% increase in distance per week, both on any ride, and total for the week. Some variation in this, more variation between individual rides than of the weekly total. The idea is to try to force the beginning rider to take it easy. Easy to overdo it early on. Most important thing is that it be fun. As you go on, your idea of what "fun" is may change.
10% interest compounded weekly adds up quickly.
It's a good idea to think about some local route which might involve hills and see if you can bite off bits of it. It can take a while just to be able to climb one hill all the way up.
Right now, cycling is the best exercise you can do for cycling.