You are stiff, so am I.

I'm in PT at the moment, and when I started the therapist said I was rounding my back at the bottom of a squat.
It's a lack of flexibility in the lower leg/feet. I suspect you have it as well.

Couple thoughts, you sound like you need more flexibility overall, time to make stretching
a big deal.

There are lots of alternative exercises you can do while getting more flexible.

Here's some shoulder therapy exercises I like
http://www.youtube.com/watch?v=A0ONH...eature=related

Stretch after you ride, right after. Start by swinging your leg in increasing circles in from of you and then off to the side.
Then do a several leg stretches concentrating on hams, quads, calf,soleus and a couple for your feet.
Get Stretching by Andersen if you need help.
http://www.amazon.com/Stretching-20t...3303144&sr=1-2
One stretch I do is I put my foot on the kitchen counter. I then lean into it, putting my foot further back each time.
Go easy stretching, patience is the key here.

Hard to suggest a leg workout because I don't know what you can handle. Step ups are safe,
and good enough that Lance does them. Keep the knee stable or lower the platform or do something else...

Lean against a sofa sitting on the floor. Put a weight in your lap and then do pelvic thrusts slowly.
You want the torso straight at the top, and don't go all the way down, just go back and forth.
When you get better, do them with one leg.

One last suggestion, get a trainer to go over technique and teach you the basics.
It's cheaper than Doctors, trust me on that one.

PerformanceU on Youtube as some great exercises, and a good video on how to squat.

One last thing, and I've made the same mistake myself, the bike is small for you.
Eventually you will want one that fits.