actually I saw the title and thought of my sister...but thats another story.
I too was in that spot of not being able to make the mailbox, and tripping and falling before my heart rate could get up to speed. I did a cardiac stress test, and they were amazed at how quickly my heart raced up to level. I only took 3 mins on the tread mill, and they had their results. I was worried I would need a chemical stressor.
I got one of those mini trampolines, and started walking in place. I got one of those bike pedal things that sits on the floor, and started biking while watching TV. I got those rubber band things, and tied them to the door knob and did some stretching. I made the effort to get to the mailbox everyday. the walk was sweaty and exhausting. Slowly I worked up to walking across the street, and then back to the mailbox, and then home. I would get more comfy with that, and would extend the walk by another ten feet in any direction, and then back home. on days when I felt I just couldnt, I didnt try.
Please dont beat yourself up about your shape, size or stamina. Take lil tiny bites out of an exercise routine. if its too hot, then find something inside to do. Lift soup cans from the cabinet while watching TV. Tie a exercise rubber to the coffee table, and pull and release while sitting down. then tie one to your leg. Dont aim for 30 minutes. that is way to much. Aim for 2 minutes. if you do well, tomorrow go for 3. Then for 4. Before you know it, you will be doing 5 minutes of stretches, and then will work on 1 minute of walking. then 2.
Then, come back here and get some cheerleading, and pats on the back. If you can do it hurray! and if you cant, hurray! we are gonna be cheering for you because you tried.