Tips on exercise

I know not to rush in to exercise postpartum, but this is my third baby and the most unhealthy I have ever felt. I am the heaviest i have ever been and i tried to do my elliptical workout i used to do and got about 7 minutes in before I gave up due to exhaustion. I used to consistently do at least 20 minutes on an incline, sometimes 30-40. Any tips on how to get back to working out or exercises you have been able to do to work your way back to a more typical exercise routine? I am so out of shape i am scared to sign up for any exercise class since it is a struggle for me to even walk to the mailbox at the end of our street and back. 😭