Hello! And welcome to the world of cycling and tri's! Good fun!
I live in the DFW area and I have been cycling here for a good 2 years.
I belong to the Dallas Athletes, and if you are a SERIOUS racer, then they might be a group you want to sign up with.
I only say SERIOUS... because they are all VERY FAST! Even the "no drop" rides are a solid 16 mph average. And, they will drop you, if you can't keep up. Maybe that sounds fun to you... or you know you will fit in perfect with that group... if so, I strongly urge you to check them out. They also run some great Tri 101 and 201 programs starting in Jan/Feb.
Now, for me... riding with them...wasn't fun (aka: getting dropped in the middle of no where on country roads is not fun). I'm a bit more casual about my training and I like to have some rides where I hammer and some where I cruise. I also like being around people of all different skills levels.
I only ride with my cycling club these days:
http://www.greaterdallasbicyclists.com/
They are a really great bunch of people... and you can find rides that cater to all different levels. You have the hammer rides, and the cruising rides.
If you go to the website, you will find the rides listed there. Now, the average they post is what you find on your computer at the end of a ride. It's not your top speed. So, to average around 15-16 mph, you will typically ride 18-20 mph and then slowing down for stop signs/etc. is factored in and drops the average.
For an introduction to the group, I strongly urge you to go to the Sunday Roll ride. It is no drop and a 13 mph average. Very casual and it will allow you to meet some people. You can also ask about other rides and figure out what rides will fit your skill set.
You don't have to be a member to ride with them, but the membership dues are $10 a year. Not a big deal.
You can also look up the Plano Bicycle Association. They have some great rides listed as well.
Once you start looking, you will find that there are tons of cyclists and ride options in the DFW area. GOOD LUCK!
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"