This is important to keep in mind. LSD has benefits, but those of us who ride long distances that include hills get interval training in, whether we are trying or not.
A ride to my parents' house is 35 miles one way, elevation gain of 1568 feet and elevation loss of 628 feet. Right now, it takes me about 3 1/2 hours to do it. Normally I take public transit back home, with a couple of 10-15 minute rides to get to the train and to get home from the train.
I make no conscious effort to do anything but ride steady, at a pace which many around here would consider slow, but I'm an old man at 53, so I'm happy to go slow. But the effort is anything but steady. There are 2 pretty good climbs that take me about 25 minutes each, and neither is a steady climb, so effort increases and there are stretches in those climbs where effort is pretty serious. There are other short relatively easy climbs along the way, but those get my heart rate up for a brief period. When I know I need to speed up to time a signal light correctly, I will speed up, sometimes significantly, and nice short burst that gets the heart rate up. I don't do this because I am wanting some kind of interval training. If I don't speed up, then my effort still won't be steady, because I'll be stopped. And indeed, there are times when I am stopped at a light, and my effort briefly goes down to nothing.
I've got plenty of other routes I typically take, from 35-60 miles, and all are the same in that the hills, and occasionally the stoplights, mean my effort is not steady, without me putting any effort into making my pace be something other than steady. Ride long and slow on routes that include hills, and the effort will not be steady, because, as you say, hills are nature's intervals.
Although the hills have the benefit of increasing my heart rate, unfortunately, the nasty hills decrease my miles per hour AND decrease my smiles per hour. I haven't yet figured out how to smile through a long nasty climb. And while I might be inclined to smile during the fast descents, I try to avoid it, because I really hate it when the bugs get in my mouth.
