How do I stop building so much muscle?

"All this muscle" is not slowing you down - even if it was, there isn't a whole lot you can do about it and still eat enough to support your training for race season.

FWIW - are you actually measuring your legs to track their 'growth' or are you just eyeballing it (or do your pants legs feel tighter?). At a very low BF% (which for a woman, 13% is very low), the muscles in your legs would look much more pronounced than at a higher BF%.

But, to answer your original question? Eating at a calorie deficit witll pretty much keep you from building much noticeable muscle. 

I would stop working out and eat more fast food.  That seems to slow down muscle growth for me.  

Really, I cannot imagine muscle slowing you down.  At 102, with some decent muscle you should have a nice power to weight ratio that will probably help your speed more than hurt it.   (And you SHOULD have ripped calves if you run 70 miles a week.)  Your body is building up the muscle in response to the stress you are placing on your body.   You are training hard enough to generate muscle growth that your body needs.   If you do not want to build muscle, stop training at the level you are and drop the intensity down a few notches.  For short distance tris, you should not need anywhere near 70 miles a week.   Maybe 15-20 at the most

You could sit on the sofa eating Cheetos until the muscles go away...

I do not know much about body building or power lifting but I do know that lifting heavy weights will produce big muscles and lifting smaller weights with more repetitions will produce lean muscle mass.  Before anyone can tell you what to do to avoid building bulky muscles, you have to tell us what you are doing to produce them.  What does your training consist of?  Do you train for high cadence in a smaller gear while cycling?  Or do you pick the highest gear and grind away like a track sprinter.  Do you pace yourself on the run or run all out?

1.  Eat more.  As stated above, a little more bodyfat and the muscles just won't be as pronounced!  Snack on healthy high-fat stuff:  nuts, avocados.   Avocados have something like 5 times the potassium of bananas!

2.  Hang out with tri guys.  As stated above... well, you got the idea, didn't you?

 Enjoy your health, girl, and have a great season!

Learn to love them?

If you aren't doing strength training, your body is adapting to the stresses (ie training) you are placing on it. To be better prepared and adapting to the stresses it is adding muscle which will make you stronger/better at what you are doing? Don't you want to be stronger and faster? I sure do!

I personally love the way my calves look and my kids laugh at me when I move different ways to show off. I think I'm kind of medium for a women for building muscle. My lean body mass is 126 and I'm 5'5". I know women my height that weigh what my lean body mass does. I could stand to lose weight but I don't want to lose muscle. It makes me faster, helps me avoid injuries and now that I'm 45 will keep me healthy longer.

Judging by your training schedule, I would say that you are doing everything well.  Your running although a bit much should keep you lean.  Your cycling seems okay but you do not mention your cadence.  Perhaps you should incorporate stationary cycling on a trainer and do an hour of TT training per week.  You should talk to a coach who could put together a training schedule to suit your personal needs.

If that is your photo in the avatar, you look very fit and very strong.  You say that you've put on more muscle in the last few years.  Unless you walk around like Bluto, I don't see that as a problem.  Those big calves you say you have would be great when running or cycling up hills.

Stop using all those mysterious products that your trainer assures you are just flaxseed oil.

Ditto.

The comment to which you responded was ill thought out at best.

AB, maybe we should go back and take basic biology?