I am living at home for the summer before I start a new job in the fall. I usually have a gym membership but I do not feel like signing up for one for so short a time. I live in Statesboro, Georgia and would consider riding with the bike shop guys in the mornings, but I do not think I could keep up with my mountain bike. My bike is very nice, I have had it regularly tuned for the five years I have owned it - it is a Schwinn. I bought a mountain bike because some of the places I ride have sandy dirt roads. I originally bought my bike to commute to work about 8 or 10 miles away, depending on which job it was. Now I have no reason to ride my bike except for pleasure and exercise.

Should I just go exploring around the roads of my college town/farming community area?
How fast should I be going? mph?
What are some workout goals I could set for myself?
Should I look for dirt roads specifically, to use my mountain bike appropriately?

I do not know alot about bike riding technically, all I know is I pretty much ride the same way I did when I first learned - just for fun. Now I would like to know where I am in the bike fitness scheme of things, and use it for my workout. Could someone tell me what I could do to challenge myself?

For starting out, just riding around the neighborhood for fun is probably the best thing you can do. Get yourself a heart rate monitor. As a suggestion, you could ride for maybe one hour at around 70% of your max heart rate, 4-5 times per week. Do a search about the heart rate monitor, there are plenty of threads that tell you how to use one and how to determine or at least make a good guess at your max HR. There are a lot of books out there that can help you construct a workout for yourself. Go talk to some of the "bike shop guys" and see what they recommend. There might be a beginner group ride that you could join.

Don't worry about using your bike "appropriately". Pushing the pedals to make it go forward is appropriate use. Ride on whatever surface you want.

I know I can ride at least fifteen or sixteen miles per hour right now. Could someone tell me what would be a good goal for me to work towards for my bike and age group (early twenties)?

Speed is not a very reliable measure since it is so dependent on the conditions (Wind, road surface etc..) I would suggest using a bike computer with a cadence monitor and work to maintain a certain cadence. Once you are able to maintain that cadence, work to maintain the same cadence while pushing a slightly harder gear. Most people try to maintain a cadence around 70-80rpm, so that might be a good place to start. You want to be working hard enough that it feels hard but you should still be able to talk in short sentences (i.e. not gasping for air)

To Crinolin:

Hey I read your post and I think I have a great solution. I found this website: exercisestogo.com and they have very specific cycling exercise routine animations you can use to build your strength. I hope this helps you. Rick