I wrote about the best cities for walking and deterrents that keep us from walking. I listed weather and hills as two deterrents.
When one is a 'beginner' at this healthy walking, it takes some time to build up heat tolerance. But, it can be done! St. Louis has very humid summers. I'm not an early riser, so my outdoor walking often occurs in full sunlight during some of the hottest hours of the day. That used to really bother me. No more. I have built up my endurance for walking in heat and humidity and find myself much more tolerant of the heat. I sweat like anything, but I can keep going for my full three miles with temps in the 90's and high humidity. It doesn't look pretty - but it feels great when you hop in the shower.
Same goes for terrain. I started my walking 'life' on fairly level terrain. My subdivision didn't have many hills and I could walk a good distance without encountering much of a grade. Once I lengthened my walks and got into the groove, I began tackling hills. Now, as with the heat tolerance, I am able to walk up fairly steep hills and keep up a steady conversation. I get winded, but I don't run out of steam.
Hills are especially good for our hearts. But, they are not the first or most important part of our walking work-out. If one never tackles hills, that's okay. The main thing is to get out there (or in there) and put one foot in front of the other for 15-20 minutes every day. More if you can.
If heat and humidity are issues for you - and they may well be enduring issues if you have health concerns - then stick to a temperature controlled environment like the gym or a mall or your living room with a fan blowing right on you.
There is no one right way to do this. The only wrong way is to not do it!
Remember my 100% GUARANTEE. Should you decide to stop walking and resume your old habits, I personally guarantee that you'll get back 100% of your former life - your pain, your lifestyle, your attitude.