I don’t ride any organized rides, but I do ride centuries and double centuries solo via my portly (36, 35, & 32 pound) mountain bikes with 29” x 2.1 and 26” x 2.0 street tires. I only rode my first two centuries via my lightweight (21.4 pound) Trek 2.1 road bike with 700 x 28 tires, then tried my 35 pound Trek Wahoo 29er hardtail on the same hilly century and found that I enjoyed the century ride much more per its greater ride comfort and its lower gearing was far better for my longer steeper ascents. I next rode the same hilly century via my upgraded 32 pound 1995 Giant Rincon rigid steel frame mountain bike and also enjoyed it much more than riding my lightweight road bike. I’ve been riding my longer distances via my portly mountain bikes ever since.
In early July I have a 250-mile ride planned (“to and from” my dad’s house in the mountains) and I’ll be riding my 35 pound Trek Wahoo 29er hardtail. It currently has 29” x 2.1 Continental street tires which provide an incredibly smooth ride and are absolutely fantastic at descent speeds of nearly 50 MPH, so even though I purchased a lighter set of tires for that ride (700 x 32’s) to reduce overall energy expenditure, I just may stick with the 29” x 2.1’s, as I intend to breach 50 MPH down one or more of the route 125 mountains and quite possibly the route 501 mountain.
Incidentally, I’m 58 years old and have been commuting everywhere via my portly mountain bikes for the last 3 years now. I only ride my lightweight Trek 2.1 road bike as an occasional treat (usually on 50 mile rides or less, as I don’t find any of the hand positions on drop bars comfortable).