Do different types of running determine muscle size?

Question
Hello Rick!  I've been running for 10 years.  My problem is a vain one!!  My
thighs are SO big from running!  I don't know whether to run sprints or to
slow down and go for time and distance.  I change my workout according to
the seasons: summer I run 5 miles 3-4 times a week at 8:40 mile/min pace.  
Winter I do a lot of interval to break up the tedium of the treadmill.   I
ALWAYS stay away from inclines!   I should add that there is no fat on my legs
- they are all muscle.  I'm 4'11 and weigh 94 pounds - my upper half is very
lean - it's just my large, muscular legs that upset me!  I've been doing Pilates
and that has helped.  I am hoping there is a certain schedule/pace/distance I
should be following to build LESS muscle!  Thank you very much for your
time!

Answer
Not sure how to answer this, as I feel that legs can only build up so much from running, and they do so by adapting to the exercise demands placed upon it.  What I'm saying is that if one was to 'body build', they wouldn't pick running as the choice to improve their leg size.  However, in some people, the legs on a runner will increase in size just due to the training adaptations.  If anything, I know the fastest way to lose muscle is to cease working out so much.  If you think of it in reverse: most sedentary people have less muscle mass because of a cease in activity or because of very little activity.  When one exercises a lot, the fat decreases and the muscle mass tends to increase as a result.  If you lift less and run a bit less, perhaps the size will shrink as well.  I can't guarantee it, but there are no 'set in stone' methods on losing muscle through a schedule, certain pace, nor certain distance to cover.  Basically what I can only think of is to cut down the number of workouts to decrease the stimulus in muscle growth if that's happening. So, perhaps try to cut down the mileage and the workout frequency to just 3x a week, and mileages to 3-4 miles. That's the only thing I can think of, really.  

I hope this helps you.

By the way, since you are a runner, I would like to invite you to sign up for my free Virtual 1-Mile Race, held on 4.18.09 through my site, http://1MileNation.com Please visit the site to check out the information on it.

Coach Rick