Sounds like your heart is with the older black lab, come rain or shine, but as far as overweight labs...
Labrador Retrievers are prone to being overweight.
I had a friend who had to leave her 5 yr old lab/mix with us for a year. She was very overweight and underexercised.
I worked w/ her and restricted her dog food a bit, but not too much. (It doesn't take much, just time and patience), and she trimmed down like a champ! Labs also love to run, so frisbee tossing and bike riding w/ Roxy running beside me, really got us into a positive routine. The pounds melted off. This girl was slim, muscular and more alert, afterwards.
A side note-
The lab's prior owner, being concerned w/ overweight, gave her iams diet forumula. I personally, don't believe in diet dog food so, at that time, gave her iams normal formula. Since then I have been educated on raw feeding, and do that with my own dog.
If, for whatever reason, it doesn't work out with the older black lab, consider the chocolate lab, or both. I wish i could adopt one of them.
Anyways, whatever happens, you can handle it, hun!
Zoey