Muscularity and Running Fast?

Yes. You will be built similar to a 100m - 400m sprinter. Now "fast"?

But right now my 5k PR is 23:30 (7:30 pace) and 10k is 49:00 (8:00 pace).  I'd like to bring them both down pretty significantly, say 19-20 for the 5k and 41-43 for the 10k. 

Not to be insensitive, but my experience and the law of averages says at that weight it will be highly unlikely to reach those times. (6:15min/ mi - 5k and 6:30min/ mi 10k) You are talking about dropping more than a minute per mile in your pace.

Once upon a time, I ran competitively in college and have run sporadically over the last 25 years. I have gone from mid-distance to rugby to triathlons over the span and the following is what my times have been during those three sports.

150lbs (8% body fat, age 20, 6ft ht) - 6min/ mi training pace - 5:30min/ mi race pace [800m/1500m runner]

190lbs (12% body fat, age 28) - 8min/ mi training pace - 7:30min/ mi race pace [rugby]

210lbs (20% body fat, age 40ish) - 9min/ mi training pace - 8:30min/ mi race pace [fat, old triathlete]

Now, everyone has their ideal size when it comes to running. Some can carry more weight than others but to most distance runners having an upper body means carrying unnecessary weight. The size and speed you are talking about is on par with higher end level decathletes; which are freaks of nature.

Anything is possible but you can definitely help your odds by getting rid of some weight to at least somewhere around 170-175lbs. If you want to keep the muscle, then you'll have to get your body fat at pretty low levels.

I'm 6', right around 170lbs and I'm pretty close to your target times. (20:43 5K - set at a sprint tri, 43:49 10K about a year ago).  Went down from about 185 lbs 2 1/2 yrs ago prior to taking up all 3 of the tri sports (no endurance background and was primarily a fast-twitch guy at that point).  I don't plan on losing any more weight (I will go down as a few lbs as I get closer to a race just from the training volume) and I have been able to continue to slowly get faster.  I still have some body fat to lose anyway so I can lose 1-2 lbs of that and be able to put on a little muscle (hopefully this winter, and planning on getting some more leg strength for the bike next year).

After about 400 meters, upper body mass isn't your friend.  Your body fat is in an excellent range so dropping weight will probably mean dropping muscle mass.  There was a general rule of thumb indicating that you could drop around 3 sec/mile off your pace for every pound loss, but this probably applied to those in the 20%+ bodyfat range.  Don't know how your training is, but maybe you can drop your times by following an aggressive training program like the Daniels Formula.

At 175 lbs my 5k PR was a 20:19 and I didn't really train intensively. Body fat was at about 14% and I was only running 4 times a week or so. My 10k was a 44:12

so its definitely possible. Just do maintenance lifting so you don't lose too much mass and focus your training towards running and the time will shave off. However you'll probably lose 5lbs or so

I get updates on mine using a tanita scale that measures bioelectric impedance.  Far from accurate, but it does give you a solid idea of if you're gaining or losing body fat.

My wife teaches ex phys at a local college so I've also had mine done using a hydrostatic weigh tank and a bodpod. . .both of which are much more accurate and precise, but spendy.

Using the tanita I've come up as high as (fluctuates quite bit due to hydration levels) 10-11% and the hydro and bodpod had me at ~5-6%.  If you use a tanita, come up with a system for when you use it, and try to keep your hydration levels the same each time.

@Tracker, I thought a 40-42 10k would be more in line with your other times, which are great BTW.  Very close to mine.