To answer my own question (in case someone searches for this in the future):
The myrtle beach area is VERY flat (no suprise) and for the most part -- shoulder-free (doh!).
So, I did two rides, one was 25 miles or so, the other was 50 miles but included a couple of hair raising miles where I had a 1.5 ft area to ride on with cars flying by at 45 mph+
I was at 77th street in Myrtle Beach proper, so I headed down Ocean Blvd, and cut across to Grissom Pkwy at 10th st or below.
I did this loop and went up and down some of the cross streets. Grissom Pkwy has a large sidewalk, but I rode in the road with no problems. It is 2 lanes on both sides, so you can stay in the right lane and people can get by.
Oh lordy. This time, I took Ocean Blvd all the way down to business 17 at the airport. I took a left on business 17 (Kings Highway) and this is the killer road. Straight, flat, and shoulder free with a posted speed limit of 45 mph. I don't know how smart it was to ride on this....but I did. You have about a 1.5 - 2' space beside the road to ride on, but thats it. I passed the state park (on the left side of the road) and took a left on 16th avenue following signs for Garden City/Surf Side Beach, etc. This took me back down to what was again Ocean Blvd. I followed this along the water until I got to a gated community where I would have had to get through a guard shack to go any further.
I then turned around a retraced my steps. You can add some miles by spinning around Myrtle Beach State Park, and by looping around the grid streets when you make it back in town.
To sum it up:
I never felt that comfortable riding in this area - lots of beach traffic. Go early or forget it -- especially on Ocean Blvd in Myrtle Beach. They have a bus lane that you can hide in - despite it being bus only after 4pm - most drivers appeared to think that meant it was 'Bus Only' all day - so you can hand out in that lane without much hassle.
The shop mentioned in this link was nice - very nice people. They have a lot of rides going out daily - listed on their web site under their cycling club -- but they had some pretty high average speeds listed. Of course - I was thinking of my average speeds when I have hills to tackle
If you need more advice - the shop has some great routes, inlcuding a metric century, and you can PM me and ping my fading (already
I am in training to ride the SeaGull Century in October with Team in Training (http://www.teamintraining.org). PM me and ask how you can train to ride a century and raise money for a great cause !!