Do you mean phosphorus (phosphoric acid)? Potassium is usually a good thing (unless you have certain heart or kidney conditions). Soda intake depletes potassium.
I’d like to echo what others have said about upping the intensity of your workout a bit. I don’t know what your starting point was, but if you’ve been walking 3 miles a day for 6 months, it’s probably time to either increase your distance, or increase your speed.
If walking/running is your main form of exercise, might I suggest eschewing the treadmill and doing it outside? I guarantee the fresh air and changing scenery will make your workout more enjoyable. I ran a half marathon (13 miles) last Saturday and somehow managed to smile at the finish line, but I seriously doubt I could run 4 miles on a treadmill. I would die of boredom after 10 minutes.
I meant sodium, sorry
Outdoors would work if it weren’t going to be over 110 degrees pretty quickly here. And I don’t relish walking in the dark when it’s over 100 at 3 am. 
Today I did the intervals of a 7 or 8% incline alternating to 4, and a bit faster, letting myself get used to the incline. (I have actually 2x fallen on a treadmill, believe it or not, and it’s fairly frightening) Checking calories expended at the end of 1/2 hour, I’d expended as many as the other workout, leaving me 15m. to do weights. I only have 3/4 hr. to do what I can.
Cautiously optimistic.
Stress your muscles a bit, I would start with dumbbells maybe.
The OP wants to maintain a certain speed at different inclines. He can do that only on a treadmill. I sometimes run 6 miles (an hour) on a treadmill, and I use the treadmill for one of two reasons: (a) inclement weather, or (b) I want to maintain a certain pace, which I am unable to do by myself on the roads. It is true that running outside on a nice morning is certainly more enjoyable, but I sometimes eschew the joy of doing that for the workout the tm guarantees.
This, and people often underestimate how hard they are working when they work out. I used to go to a gym and power walk on the treadmill, a woman stopped and gawped at me for long enough to make me feel awkward and I came to a halt and asked if I could help her with something? She mimicked the way I’d been walking “is that how you’re supposed to do that?!”
I asked her to demonstrate what she did on the treadmill, she put it on an incline and a moderate speed, but she wasn’t making any effort as she was walking, she was keeping up with the speed of the treadmill, but she was leaning on the bar on the front of the machine. I told her to take her hands off the machine, and swing her arms, all of a sudden she can’t keep up with the speed of the machine…
Several weeks later she told me she’d lost 10lbs now that she was actually walking on the machine.
YMMV
Ha, yeah I can certainly respect that.
From walking? Walking doesn’t build much muscle.
Maybe use the 45 minutes to do something else entirely? Alternating between jumping rope (which is hella-intensive cardio) with dumbbells/weights, for example. Or doing jumping jacks, pushups, etc.
Strolling doesn’t, but walking 4mph up an incline will certainly build muscle in your legs at least, if you were sedentary before. Not like weights, but enough that if you stay the same weight, your waist should be noticeably smaller and your calves bigger.
If you are concerned about the tiny amount of sodium in a can of soda (1-2% of DV, meaning 50-100 cans a day), then you should be much more concerned about the sodium in prepackaged foods; regardless of how healthy they are in other respects (I see some that look pretty good except for the sodium), virtually all processed foods are simply loaded with sodium, as if they need it to preserve it or something (which is untrue for most foods, especially canned and frozen food; after all, they couldn’t claim no preservatives if so). Of course, if you aren’t sodium-sensitive, it isn’t as much of a concern but you still want to limit intake to no more than the federal guidelines (I am not, but I still get reduced sodium over regular, plus it replaces it with potassium (taste difference? I don’t notice any), something many people don’t get enough of).