Hmmm...sounds kind of fishy to me. In general, skinnier cyclists are much faster going up hill. At 5' 9" and 240 lbs, you are far into the "Obese" category. If you are consistently the first one up the hill then I can assure you that you're riding with a pretty slow group. And if you were to lose a bunch of weight, you would undoubtedly get much faster on the bike (except, perhaps, going down hill

).
And, FWIW, it sounds like your typical route has about 35 feet of climbing per mile...that qualifies as "rolling" terrain, not "hilly".
By way of comparison, I'm 7 years older than you, and my ride yesterday was 63 miles, with 3500 feet of climbing (56 feet of climbing per mile). With only a couple of stops, I averaged just over 18 mph solo.
As for training techniques - right now I'm just building base and getting back into the bike. I've only been on my new ride for 3 weeks, after a 4 week layoff due to being hit by a car while I was riding 40 mph down a winding canyon road.

I employ a mix of "old school" (ride lots, ride hard), with "new school" (structured intervals, hill repeats, diet, etc.).