I did this to the max. I rode on the Nor-Pac trail from ID 10 miles to get there (75% downhill). Then the 15 miles of the Hiawatha in MT (all 2% downhill), then back. The Hiawatha trail has 10 original train tunnels and 7 high trestles. the tunnels were dark, muddy, with water coming down in random places. The trestles have warnings and lots of rotten wood on top, but the supports are concrete and metal (rusting). The longest tunnel is 1.7 miles and freezing all the time. You can feel the woosh of cold before entering. This was one of my bucket list items and is going back on the list to repeat in 5 years! I thoroughly enjoyed it! The hardest part though, was the 10 miles to get there and return. This part was rough and higher inclines. WHAT A DAY! Every cyclist should do this one!