I'm running 40mm Schwalbe Marathons on my Haro 29er. I've been up to 85 miles a day on them. They roll reasonably well but are still robust enough to resist the pothole rich roads I sometimes find myself on. As an off topic aside, I assume you're wearing MTB shoes with cleats, if so, check out the Shimano PD-280 "Saint" pedals. They have a much larger platform surface area and I find them to be very comfortable on longer rides.