I've been able to do 2 1/2 hours without too much boredom.

I've found that watching the right thing makes the time go by quickly. I watched La Femme Nikita during my first winter of real training, and Highlander (TV show) during my second. This year it's Buffy the Vampire Slayer.

Having specific workouts helps. Pure endurance pace is fairly dull, but throwing in tempo intervals or bursts or something helps.