MPH advice
 

I'm still new at this biking thing. I have a trek 4300 MTB that I got to ride around town to do errands and have some fun on. I've had a string of dehabilitating injuries so I am unable to run. I've been biking out on country roads. There are some smaller rolling hills but nothing too major. On my longer rides I average between 15 - 15.5 MPH for 45 miles. I don't know how that compares to a road bike. I think I'm doing well since I've increased about 2 MPH since I started. I would like to know if this is a good pace compared to a road bike.

It is a good pace for a MTB.
A road bike would be quicker but that is like comparing apples to oranges.
The only important comparison is the one you have already done with increasing 2 mph.

In the spirit of the original question, I average 17 mph on a 28 mile commute through traffic, lights, stop signs, etc on my CF road bike. Memorial day I took an 80 mile fairly flat ride and averaged 19 mph.
To make an accurate comparison is difficult. Too many variables.

I say you are doing just fine.

What's your goal(s)? Fitness? Weight loss/maintenance? Racing? All of the preceeding?

My goal is to stay in good cardiovascular shape so when I can run again I won't be starting all over.

overall mph really im starting to find out isnt the best way to keep track of your improvements. unless i am doing an exact ride like a TT or such