LIke most UFO sightings by civilians, this one leaves a lot of questions unanswered. For example, your report is very detailed, but much of the facts you relate are unknowable. You describe a completely unfamiliar object, one you cannot recognize, and all you can give in way of description is its shape, color, and lack of surface details, which you report in some precision. So far so good.

But you also describe the object as being 100 feet over your head, traveling at 150 mph and approximately 60x120 feet in dimensions. If this had been a familiar object, such as a plane or bird, you could make these estimates of size, speed and altitude because you would know what it really looked like. But you have no way of knowing how big this thing is, so you can't tell how high it is, and therefore you have no way of knowing its speed, either. It could have been a kite ten feet over your head, or a 20 mile-long starship flying at orbital altitudes. You just don't know. You can't. Right away, this casts doubt as to the value of your observation.

Yet you feel confident enough about relating these details to a group of skeptics, so you have already crystallized in your mind what you have seen with properties it would have been impossible to detect even if it HAD been the real thing. When you see something unusual, under unusual circumstances, your brain does not have enough information to fill in the details, so it makes them up in an attempt to make sense of the sighting. You probably saw a very ordinary object under unusual circumstances, and what you remember is just not what you saw.

Please don't think I'm calling into question your honesty or intelligence, I know it can happen to anyone because it has happened to me. I consider myself the ideal UFO witness, both because of my background in astronomy, but because I had training and experience in the military observing aircraft under unusual lighting conditions.

But I can be fooled. I used to live near Moffett Field (home of NASA Ames) and all sorts of weird aircraft used to fly over my house. I remembered seeing one in particular, flying very low, about a thousand feet up, over my house for several nights in a row. I found it unusual because it was totally silent and because it was showing no navigation lights, a highly illegal no-no in some of the busiest airspace in the world.
I assumed it was some secret experimental craft they were testing. And this thing was BIG, bigger than a C5A. I knew of nothing like it.

One night it came over, but my porch light was off, so my eyes were dark adapted, and I finally got a good look at it. It was a Great Blue Heron, gliding back (about 50 feet up), to roost at Bayshore Park, over by the brine ponds near the airfield. Subsequent sightings confirmed that it took the same flight path over my neighborhood every night.

I could not immediately identify it as a bird in the dim light, so my mind filled in the details, giving it an altitude, size and speed it did not have. I think you saw something similar and made the same mistakes. Or at least, I have what I feel is a more likely explanation for what you saw: an ordinary object under unusual conditions.
Before you protest too loudly, I have other stories like this, several of which I have related here. This really happens all the time. I have seen honest perceptive people completely misjudge what they saw, while I was right alongside them with local knowledge of the true nature of the UFO, and they still could not accept my explanation.