If you just want to finish, maybe it’s enough.
If you want to do your best, it’s not enough. Many times I’ve seen people doing a long ride with a lot of climbing…maybe they’ve done a lot of trainer time & have pretty good fitness…but at an appreciable intensity & a duration they have not been exposed to…hitting muscles on hills at angles that they have not trained…now all of a sudden they are getting cramps. ‘I usually don’t get cramps!’
For sure, that person has been me in the past!
Do some long rides. Use them to figure out your nutrition. Figure out if your wrists are going to hurt. Figure out if our saddle is going to start killing you. Figure out if you’re going to have hot spots on your feet. Use those long rides to mimic the climbing you’ll be doing. Do you have to? Maybe not. But you’ll certainly maximize your chance of success if you get out and do some long rides.