You said you haven’t done sports in decades. Give it some time and be patient with yourself. Plus, have a look at this graph: 2.7 W/kg puts you right around the average of all male TR users — and you’re doing that practically off the couch. You are not as slow as you think. Imagine what you’ll be like in a year with, say, the exact same power numbers, weighing another 10-12 kg less. You will feel that when you are outside, especially when you are climbing.
One thing about nutrition: when I am in my off-season my cravings shift towards unhealthier food. And when I am in training, I actually want to eat healthier food. So just training will get you into a virtuous cycle. I don’t know what your diet is like, but two easy ones are sodas and beer. I share a bottle of beer with my wife about once per week. Of course, sometimes I drink more when I am out with friends. But other than that, I have cut back on that. Sodas are 100 % gone in my life, unless I am riding outside and need the energy.