People have ridden far less "optimal" bikes successfully on endurance rides.
The big question in my mind is one of desire - Do you want to ride a 20-year old bike badly enough that you're willing to do the maintenance and parts hunting required to keep it in good shape? And are you willing to take the time necessary to learn all of the bike's oddities?
Nothing wrong with old bikes. Just don't be lulled into thinking that everything is perfect and go in to it with blinders on. Be ready to do the work (literally and metaphorically).
I rode a 1984 GL1200 Aspencade on more than one very long ride (I even did an IBA run on it). But it took work, discipline, and a willingness to get creative about sourcing parts that were NLA.