Personally, I don't think anyone can really translate it into some set distance but rather hours spent on a bike. The speed at one travels at varies too much from one rider to another and on the terrain/road condition one happens to be riding on at the time. This would apply to any style of bike that one is using.
Since you are essentially rediscovering bicycling, I would suggest riding short distances as much as possible, not only to get used to your bike and what it is capable of, but to build up your muscles and endurance. Riding a few miles around your area, if not daily then as frequently as possible, is a good start and if something happens (bike needs major readjustment, flats, mishaps, etc) you are close enough to home to walk you and your bike there. It is also a good way to find out if your saddle/seat is up to the task the longer (timewise) you spend riding on your bike in relative comfort. I find many women prefer a seat with a shorter "nose" or length, a seat width slightly larger, and some kind of memory foam padding for more comfort.
The flip side is to ride as far as you feel physically capable of and stop to rest. You are the only one that knows your limits and when it's time to turn around and head back to your car/home. The key thing is to try to not to over do it. Don't forget, the older one gets and the longer the planned ride is, the more liquids you need to stay hydrated...which may translate into a few pitstops along the way.
After saying all that...lol...although I don't own a hybrid, I have used my cruiser bike, which is as upright a riding position there is, on 40-50 mile rides (round trip) without feeling that my seat is assaulting me or having an excessively sore bottom at the end of the day. But that's just me and I do lean/hold on the middle of the bars with my elbows on the grips on relatively long straights...lol (not recommended for safety reasons)... as well as using different hand positions on the handlebar. Plus I have a springer saddle with memory foam on that bike to absorb the bumps and provide more comfort (actually my significant other changed the saddle/seat since she uses the cruiser most of the time...lol).