Weight and practice are huge. When I started riding seriously in Placitas, NM (foothills of the Sandia mtns at 6-7k ft) the biggest obstacle was - I wanted to ride 10, then 20, then 30, ++/etc miles, but EVERYTHING was either a 5-15% grade up or downhill -- no flats. After struggling with a rollercoaster ride, w/heart rates alternating btwn 160-170s to resting downhill for a couple seasons, 3 things happened. The 3rd was the big surprise. 1, I gained a lot of strength. 2. KEY: I lost a bit of weight (with #1, a big increase in strength:weight ratio). 3, I started to enjoy! riding hills.