Completely agree-and im speaking from a sympathetic experience!
I was 19 stone and 9 pounds at christmas (you wont be any fatter than that id bet)
Now 17 stone and doing 80 milers.
I do spend a lot of time training now but my top tips would be.
1)Get a comfy saddle and ensure the bike is fitted for comfort. (ie invert the stem tilt bars towards you etc)
2) Dont scrimp on shorts. DHB's are reasonable, a good chamois will make you get back on the bike because you wont be waddling!
3) Use chamois cream. (i use assos). The bigger the thigh/crease the more important this can be!!!!)
4) Take it easy and build up TIME in the saddle 10 minutes at a time -dont even think about speed/distance for the first few weeks.
5) Most importantly enjoy it. Dont starve yourself as your body still needs fed but try and eat healthy foods and drink gallons of water.
6) Ride as often as you can but to start with leave a rest day between rides if you are untrained. I now take one off in 8 and feel great.
7) Try your local club for tips/encouragement, its helped my motivation.
Good luck. My body fat has gone from 34 percent to 21. Im still loosing weight ( gonna try for another 18 pounds)and life couldnt be better.

BOSH