I also ride in rolling terrain where there is almost nothing that is flat and nothing goes up (or down) for very long. I got back into century riding last year where I rode a local century (total of around 4500 feet of climbing and NEVER more than a couple minutes at a time) and the Six Gap Century which is around 11,000 feet of climbing.
I did a lot of riding in my local area simply attacking all the 'climbs' as I encountered them on local rides. The local century (4 weeks before Six Gap) was treated as a training ride. My sense of things was that the training that I did left me well prepared for that ride.
A month later I rode the Six Gap (late Sept) and finished it but had some leg cramp issues and my sense of things was that the training that I had done did NOT leave me well prepared for that ride. My legs simply were not ready for 30 to 50 minutes of constant effort because they had never done that before.
I am going to ride the Six Gap again this year (before I age out of the event as I am already in my 70's). If I had LONG stretches of level terrain I feel that I could simulate climbs with appropriate gearing. But that does not exist around here and the constant (although short) downhills just mess up any effort to simulate a climb. So my plan this year is two fold.
1) I will occasionally be going out for maybe a one hour ride outdoors and then come inside on my trainer (which is a reasonable quality spinner bike with Garmin Vector pedals) for some simulated long climbs.
2) I have found one stretch of road where the average incline is less than 1% but there is VERY little downhill, so I can use that to simulate climbing (450 feet of climbing over 18 miles). Elevation map below.
We'll see how that goes this year.
dave
ps. Those 2 elevations are about as low and as high as anything in the 3 county area where I ride.
'Climb' route