Congrats on getting out there. In terms of how long for 20 miles. I don't think it is unreasonable to shoot for an average speed between 13 & 15 mph in your first season of returning to cycling. Build in time to pump up the tires, fill up the water bottle and get dressed - I'd budget two hours for a 20 mile ride.
__________________
A conclusion is the place where you got tired of thinking. - S. Wright
Favorite rides in the stable: Indy Fab CJ Ti - Colnago MXL - S-Works Roubaix - Habanero Team Issue - Jamis Eclipse carbon/831