How far is too far to walk for me or my dog?

I am 22 years old 5'10" and 237 lbs, but I'm still reasonably fit having been a swimmer all my life. I recently got a dog and found that I adore long walks and they are not difficult for me at all. I have easily completed 2.5 mile walks in just under an hour, and although I am breathing hard all the way through (exercise ind. asthma), they don't wear me out. I'd like to ramp my walks up to 3 miles twice a day, but I'm not sure if that's too much for me and especially for my 7 year old Cocker spaniel. Is six miles a day too much for me or my dog? Naturally I bring my dog water and I'd work a break into the middle of each walk for her.