MaryEllen,
Its great that you are feeling so good and that your recovery has been smooth but I must say I would really, really advise you to hold off on the running for a few more months. It just does not make sense to push it this early - you could potentially have the rest of your life to run on this device or you could fracture your femoral neck at 4 months post op and be done with running.
I have been a trail runner for more than 15 years and I am definitely not going to even think about running until 10 months. I will hike, bike, mtn bike a little swim, do yoga and even backpack this summer when I get to 6,7,8 months but I am absolutely going to wait until 10-11 months to return to running.
Here is a story that has given me a bit of pause. Have you heard of Robyn Benincasa? She is a world-reknown adventure racer. She does ultras and multi-sport adventure races. She had both hips resurfaced in 2008 and 2009. The 2nd hip was done by Dr. Su so it is highly unlikely that surgeon experience had anything to do with her failure at least on that hip. Her resurfacings BOTH failed within a year or so because she had stress fractures of the femoral neck which eventually fractured all the way through. She started running at 3 or 4 months...not exactly sure - you can find it on the internet somewhere. She was doing marathons again within 6 months. Well, now she has two THRs. Obviously THRs aren't the end of the world by any matter of means but I'm sure as an ultra-runner and distance athlete she is not too happy that her resurfaced hips failed b/c of her own lack of self-control.
I am honestly not telling you this to judge or to scare you. I mention it because I am that person who has been unable to hold myself back and has pushed it recovering from past surgeries way too fast...and it has caused a lot of problems.
If your body is telling you to slow down (soreness, pain, etc) than it is not ready for the stress you are putting on it by running at 18 weeks post-op.
Good luck,
Amy