That’s a lot of weight to carry around running, even for a short distance, if you haven’t been at least walking. I would suggest starting with some walking. Once you can walk for at least 20-30 minutes without a break, you can start throwing in some very short, very easy running segments. For example, walk for 2-5 minutes, then run very easily for 30 seconds, then walk for another 2-5 minutes, then run very easily again for another 30 seconds. As your fitness increases, increase the running segments until you’re running very easily for the whole 20-30 minutes.
Once you can run non-stop for at least 20-30 minutes, you are ready to start thinking about that 1.5 mile timed run. Until then, just focus on getting used to running.