While I'm sympathetic to the anxiety that you experienced and the thrill of success you felt by acting in a way that is more assertive I'm not going to praise your efforts.
You chose the wrong battle to assert yourself on, and you didn't use the momentum of the conversation in your favor.
After I collected myself
I hope this happened in 15 seconds and not after the conversation concluded.
Once the conversation ended revisiting it is admitting that you are petitioning her frame. She didn't engage you at all, and simply walked away.
At this point if this weren't an eight year relationship I'd suggest a preemptive break up. I hope you have a very good grasp of how separating your life from this woman would play out. I also hope you went ahead with your relationship rehab knowing that failure might be the end result.
http://therationalmale.com/2012/08/22/just-get-it/
This is the biggest hurdle for most guys to overcome. You are not winning a debate when you appeal to her better nature. You can win, but you've fostered resentment for winning in a way she doesn't respect or appreciate.
If you'd just let that conversation die and let her go out and experience getting shit thrown in her face it wouldn't have been a big deal. Afterwards you just shake your head in a masculine way and tell her to hose off before she gets in the car. If she wants to do it with friends tell her, "Good. Carpool and get that powder bullshit in their car."
Don't debate your opinions or why your feelings have value. You aren't selling magazines or trying to keep her from walking off of the lot. You never ever pussyfoot about what you want. If you don't want to do a themed fun run, what is your alternative? After 8 years "Stay home and have sex" probably isn't as effective as a last minute blow off from a spur of the moment romantic coupling. So what are you contesting with your woman at this very moment?
"We never have fun!"
"I don't enjoy that activity."
Hers was a general claim, yours was specific. You weren't even having the same conversation. Your assertiveness was completely undermined by missing the big picture.
If you were someone I knew in real life I could offer more detailed advice, however in my experience an accusation of being boring is rarely specious. It may be unrealistic given your life and commitments together, but probably not inaccurate. You just have to identify what she means and what it is she actually wants.
Most girls want to show off their man, or be able to brag about him. I've known fiercely loyal women that see their husband for maybe five waking hours a day while they're working. The sort of women that wake up before their men and share the time before he has to go, whether they are stay at home moms or working themselves. If you'd asked any of those women whether they wanted a quiet day at home with their partner or a fun run where they tried not to shart warm gogurt they'd probably take the quiet day.
If I had to guess what your problem is it's either you are too present, or not braggable. You have good qualities (hopefully not complete beta bucks) that aren't conveyed to her friends and family when you aren't present. Whatever that quality is it requires that you display it not just to her, but to the people whom she wants to impress.