Best National Park for Flat Trail Hiking / Walking?

I discovered hiking in my very late 40’s and with it so many beautiful state and national parks. Unfortunately, I have had multiple heart attacks, two bypass surgeries, and so much more in the last few years, with 2024 being the roughest. I am still able to walk a decent amount (1 to 2 miles max, though I could do 3-4 of these over the course of a day), but I currently cannot walk anything with more than a gentle incline of a few degrees over a short distance. What park, state or national, any where in the US would you recommend for the level of activity I am able to do?

Of course, I love, love, love the mountains (argh!!!!), but also forests, waterfalls, lots of green, lots of wildlife, and flowers and plants you don’t see often or at all in Texas. I’ve been to Yellowstone and the Tetons 5 times and they are my favorite parks. RMNP x 2. Olympic NP (loved it), and several state parks in Texas and Colorado, and Washington state.