Our hair is very similar in color, and I use #7 Golden Chestnut to blend my grays into natural highlights. If you don't mind the gold/red tones and want to turn the grays into highlights, you could use the same shade. It should not darken the rest of your hair. You can see my hair in my fotki album too.
If you want to get away from the red/gold, then you would need to mix it with a neutral. Do not use #8 as that may darken your hair, it is actually darker than #7.
I would suggest #7 Golden Chestnut, do a strand test or two on different parts of your hair (gray and non-gray) so you can see how long you need to leave it to cover/blend gray without getting too dark. You don't need to cut off the hair, just separate, apply color and wrap with plastic wrap to keep separated. Check your timing. Probably at least 10-15, it may take up to 45 if your gray is resistant (like mine). But most likely not.
You can apply the color to the strands, check the time after 10-15 min. You can just wipe the strand dry with a paper towel. If it's not dark enough, reapply mixture and check again in another 10. That's the easiest way to strand test.
Since your hair has been processed, the color may take more quickly until it's grown out. So watch the strands and time carefully.
When you're ready to color, it's best to shampoo or lowpoo instead of condition wash as that can leave a coating. The color won't stick then. Do not use styling products before coloring either. When you have found your correct timing, apply to the whole head, starting at the gray areas. If you have an SO or friend that can help, that's easier, but you can do it yourself.
When your color is right, rinse and condition wash. You don't have to shampoo, but don't use anything rich in oils, glycerin, etc as it will strip the new color. Same goes for styling products. Avoid oils, heavy creams, glycerin for at least a week until the color is really set. Do not thoroughly wet the hair for 72 hours if possible. Just spritz and style.
There are a few salons that use RC but mostly in NY. Hair Color, Hair Care and The No More Bad Hair Days Kit by Robert Craig Salon Products, Ltd.
Did I answer everything? LMK if you need anything else. I'm happy to help out.
Oh, since you have long hair, you may need 2 boxes the first time. After that, you will just be coloring roots mostly and 1 will suffice.