First Ride 15MPH Avg - I'm Way Humbled!

A bit.  More than 1mph.  Les than 5mph.  Probably.

A bit.  Articles are way less helpful than experience.  Shift to keep your level of effort and cadence more or less constant.  (That's for maximum performance.  In training, there are excellent reasons to vary effort and cadence.)

Hard to say, but the good news is that improvement early on can be very rapid, and over longer periods can be quite dramatic.  Race pace for me (now) is anywhere from 21mph to 26mph, depending on distance and conditions.  My first training ride (3-4 years ago on a road bike) was probably around 16mph.

My suggestion:  Don't focus on a desired pace.  Doing so can be both frustrating and limiting.  Focus on the desire to go faster.  You will achieve that by riding more and riding harder (most of the time).

that's why we all spend a fortune on these tricked out TT bikes.  they make you faster for the same amount of work.  I think you've got a new bike in your future, and you'll easily be able to break that 1:30.

Thanks all helpful so far!  By no means was I killing myself today, I guess I was just surprised is all.  I mean I caught and passed some slower riders who were on road bikes and twice some dudes on road bikes passed me like I was standing still.  I felt like saying, "why don't you meet me at the pool or track?"  

Egos aside, I think I can improve dramatically as I have in swimming and running.  Like a lot of competitive people I guess I just expect so much and although I'm turning 40 in 9 days I just don't look at myself that way.  

Ha! About-to-turn-40 is young around here. Welcome to the largest and most competitive AG in triathlon.

As others said, don't sweat the mph, as it's really not terribly informative as a general measure on the bike. That's why you'll see the faster bikers generally talking about power rather than speed. 15mph on a MTB on a hilly route isn't at all bad. Tires, hills, wind, road surfaces all slow you down, and riding position is huge. Learn how to train for the bike, and start making equipment & position improvements, as your motivation and finances allow, and you will get faster. In my first ride, 3 years ago, I averaged less than 14mph over 30 miles on my old hybrid; nowadays on my TT bike I'm generally among the faster riders in the AG.