Congratulations
If you ride stationary bike and got that sore from a 3-mile ride, I have to ask if you got a proper fitting at the bike shop? If not, go back and ask them to help you with the adjustments. Proper fit is 95% of comfort on a bike.
Don't get an overly wide and padded seat or overly padded bike shorts. While it sound like it would increase comfort, the opposite is true. Excessive padding just increases the pressure on the sensitive regions while distributing weight away from the sit bones (ischial bones) where it should be. If you have a well fit, appropriate seat, the only soreness from pressure should be a circular area over the prominence of the sit bone on each side. It is counter-intuitive, but a less padded seat can actually take pressure off your more sensitive areas. The purpose of a chamois in riding shorts isn't so much for padding as it is to reduce friction, thus preventing saddle sores. Good riding shorts shouldn't feel like a diaper.
Once you have a good fitting bike with a seat that works well for you, its just a matter of riding frequently adding a few miles at a time. The initial soreness will subside and soon you'll be cruising all afternoon and having a blast.
Oh yeah. Green Slime = BAD