Too much exercise?

I bought Daisy, my five-year-old black Labrador when she was eight weeks old and for as long as I can remember we have been totally inseparable. Unfortunately I fear I may have overdone it with the exercise over the years. Is it possible to have done too much walking with her and what would be the consequences?
I was ill myself at the time and didn't work for many years. To help me cope I would walk and walk and of course Daisy always came with me. Most of the walking was on tarmac roads or hard surfaces. I think somedays we would go out for four to five hours at a time and the mileage would reach 15 miles at least. I hasten to add that this was not when she was a puppy, but started when she was about 18 months old.
However, I have now noticed she often quickly turns to try and 'bite' herself on her hip area and looks at me as if to say, 'Mummy I hurt there, but don't know what's wrong'. Sometimes when she is walking her rear end gives way for a second or so, and yet then carries on as if nothing has happened. If she walks on concrete for about an hour she starts to limp and in the evenings she licks her front paw pads. She is of average weight and is a very healthy, happy, fit dog. I give her cod liver oil capsules and a high strength flexible joint supplement containing glucosamine, omega 3 etc daily.
I have taken her to the vet who is very thorough usually, but he didn't see the need for X-rays. But I know my beautiful Daisy and something isn't right. Have I caused some damage by walking her too far, and if this is the case, is there anything I can do to rectify this? I am worried I have given her a bone problem and can't forgive myself.
Joanne Bedwell, by email