Are you doing the 3/4 mile all in one walk? If so, and if your schedule will allow, I would break that up into 2 or 3 walks per day. Preferably 1 after breakfast (allow some time first for rest after a meal then walk) and one after dinner. This might help with overall stamina and since the walks are shorter per each, you can probably get more mileage from her per day.
If she has displasia, arthritis could be settling in. Natural eggshell membrane (NEM) is worth a try. You will see results in about 10 days give or take a few days. Bone broth is also great.
My old girl, a Heinz57 mix, was fat and that is what I did for her along with cutting food intake very slowly. It took a while to start seeing results. The several walks a day especially when my boy was a pup was a bit exhausting at times, but she did shave the pounds off, ended up with a beautiful tuck and what I called her bikini figure lol. She was at age 11 when I took the glasses off and realized what had to be done for her. I still smile when I think of her.
Both me and my boy take the NEM. Personally speaking it has just about stopped my morning minor aches and stiffness.
I'm sure others will come on for food suggestions. Much luck to you with this.
"If you can't see his soul when you look in his eyes, then you need a seeing-eye dog"