RIDE MORE-EAT LESS
Sounds simple but ofcourse it's not.
At 245 Lbs, my endurance sucked, was doing 15-20 miles a week off road. (one ride)
At 200 lbs and doing 50-60 miles a week off road, Endurance and speed were way up!
(That is until one highside and one well placed tree root, broke my femur)
Being married too, I understand the wife thing and the time to ride thing.
You have to ride though, endurance will never get better doing one ride a week.
Get up early, ride the MTB on the road at least twice a week and at least once on the weekend.
Or, get a stationary bike, ride for at least an hour per day, you and the wife watch tv right?
To start, ride at about 70-75% of you capability for long periods of time (one hour plus), do this for 3 or 4 months. Eventually add some sprint intervals.
* Those long slow rides will be hard to do at Alafia or Carter, suggest Boyette and Flaywoods or the local roads.
ps. I don't claim to be all that knowledgeable, but the above process worked for me.