Walking, rather than running or gym work, was what I used to improve my fitness, and it paid off.
At 115kgs, my feet hurt so badly when I first tried walking that even 5 minutes at a time was striking. I looked into a lot of shoes and insoles, and what I finally settled on was wide fitting merrell moab hiking boots paired with gel insoles, for walking along uneven gravel tracks in the bush, and they made a big difference. As I continued to drop weight, though, there came a point where my feet no longer hurt regardless of what I put on, so the whole thing felt somewhat pointless after that. Even so, they are comfortable, durable boots that will not slip on rocks, dirt, gravel or mud tracks, and they will likely hold up for years.