I just started TR about a month ago and had a similar question, because I find it ‘odd’ that TR doesn’t seem to take the course into account, only the (estimated) time. This, to me, leads to the impression that two people doing the same ride, with one estimating doing it in 5 hours and another in 8 hours, would end up in a situation with the person estimating 8 hours getting a ‘harder’ program, because they are planning to spend 3 more hours riding, when, in my estimation, the person trying for 5 hours needs the harder program.
When posing this question, the basic answer I got back was ‘it doesn’t matter, because the adaptive training plan will adjust to your fitness’. While my head is still having some difficulty wrapping my head around this, I’ve been trusting in the TR logic to design my training plan.
So far, after some initial concern (while TR was ‘learning’ my fitness level), I feel like it’s on the right track. The one place where I do feel like it will need to override the plan is to do some longer outdoor rides (as weather permits) because, while TR may be able to increase my fitness with training sessions that aren’t as long as my actual event, I feel that there are some aspects (nutrition, and just ‘being on the bike’ for an long ride) that just can’t be addressed with a shorter workout.
In the end, though, you’ll have to decide what works best for you (largely by just listening to your body). Be sure to rate your workouts accurately, as this will determine what TR prescribes for you going forward. And, as far as the specific distance to choose for your ride, just make a choice and go with it. Personally, I’d go for the 50 mile ride. Yes, it will be longer and harder, but it will also be more of an adventure. I think you might be surprised by the extra motivation that riding with a bunch of folks gives you. And you also don’t have to do the entire ride as ‘one ride’. There will likely be a few rest stops where you can refuel and recharge, and make it, effectively, multiple shorter rides.
That’s just my two-cents, and I’m sure there will be a number of other perspectives from other people. Whatever you choose, good luck, and have fun!