Agree long walks are great for burning fat without stressing recovery, especially as you get older. But I still think "doing cardio to lose weight" is a misnomer. Far more important and what most folks are after is improved body composition i.e. fat loss. Before I started doing regular squats and DL at age 40, my normal routine was bro splits for upper body and running for lower body. When I upped my running volume I would actually gain a few pounds because I would put on a few pounds of muscle on my legs. But it would incrementally lower my bf % unless I was in season for wrestling or Judo (in which case I was already getting tons of cardio from those practices).
I think most people have a genetic floor for sustained bf % and getting below that via increased cardio or reduced calories will catabolize muscle mass. But if you're above that floor, losing fat is straight forward via caloric deficit in conjunction with strength training.
Target HR as a % of max HR varies by individual, and the outdated formula of MHR = 220 minus your age was debunked due to being based on a very small sample size of sedentary people. Best way is to determine your actual max HR and anaerobic threshold with a chest strap (more accurate than wrist). Using myself as an example, in the mid 90's when I was 21, I could achieve a true MHR of 201 bpm sprinting it out at the end of a hard 5K. 26 years later, my MHR achieved during a run last week was 198 bpm. I've also found my anaerobic threshold (highest sustainable HR during a hard run) has remained 175-180 bpm since the 90's. Although my work output and running pace at any given HR has gone down over the years, my actual MHR and target zones have remained about the same.
