Gosh, I can't say I feel exactly where you're at, but as a 41 year old who has kids and is somewhat afraid I'll not be healthy enough to keep up with them... for that I'm right there with you. You would never think of sending your son out the door without the things he needs. A coat. His lunch. His science fair project. He needs you, too. Take it slow and/or consult your doctor first if you're unsure about how hard you should attack this goal.
I've seen more than one person say on this forum that if you stop, get waylaid by injury, etc: Just start over. Keep trying. You can do it. Today I did my second session on the app. Day two for me. I did NOT want to go out this morning. I had a migraine last night and alot on my schedule today, but I kept in focus that my kids need me, and they need me to start something. I'm taking ibuprofen. I'm drinking LOTS of water. I'm hoping my shins will recover by Friday. My body is in a state of shock I think!
That said, it's my opinion that the most important thing to take away from this app is routine. Start your five minutes just walking - heck, just strolling, and on the jog portion do a brisk/fast walk instead. Just move. I am so encouraged by others I see out walking/running/biking. They are all shapes and sizes and ages. They inspire me. My kids inspire me (your son will probably cheer you on and may even want to join you in this endeavor)! This forum inspires me. I'm not on MyFitnessPal (yet?) but lots of people love the accountability and support it offers.
You can do this.