I ride a 29er with 32,20.
Rides are usually less than 3 hours.
I rest for water stops and ride quite leisurely.
But I do hammer the hills religiously and spend a lot of the time out of the saddle upright pulling on the bars.
Most of the time I'm panting like crazy either at the top or after getting off and walking or running to the top.
Will my muscle mass increase given a healthy diet? Its kinda like stop start training on trails. Heavy on the hills and techy bits followed by a minute or so resting and then back on.
I've seen guys with massive thighs that dont adhere to bodybuilding diets yet they defy the industry with huge muscles.