Does it take you longer to warm up the more mileage you do?

You mean a long run vs a tempo run?

It always takes me a couple of miles to get in to a long or easy run, while the prospect of a tempo run, race or speed workout really gets my adrenaline pumping.

Also, I tend to do speed and interval workouts with a group but do long runs alone.

I notice the same thing. I always figured it was a matter of clearing my legs from the previous workout. I also think the muscles don't have to work as hard when you're in shape, so it takes them longer to physically generate heat and warm up. Just my crack pot ideas.

When I'm just starting back, I can feel good pretty much right away into a run. When I'm running 10+ miles per day, I don't feel good until 3 miles into the run, and I never feel great until 6+ miles into the run. Kind of makes me wonder what the optimal warmup is for a 5k or 10k. I don't really start feeling good until 3-6 miles into a run, but running 6 miles before a 6 mile race seems counterproductive. I think the excitement and strides leading up to races help get me warmed up faster anyway, but still.

Yep, I definitely feel that way. There are some days when it takes my legs 40-50 min just to start to feel good, but when they do it's like a switch goes off and I feel absolutely great. Especially in the summer when I first get up to 75-80ish.