I am becoming a fairly strong rider (for my age). My strengths are speed/endurance on the flats, and short steep climbs (max 2:00 minutes, up to 1/2 km long, but best at 200-400m). I can often pass stronger men on short climbs. But my endurance on longer climbs is poor. I fizzle out pretty quickly and can't maintain the speed.

My question:

What can I do to improve in this area? I'm thinking I should gradually increase the length of my hill repeats. This past year, I was mostly doing 6-8 X 30-60 seconds of hard climbing with 1-3 minutes rest in between. The longest specific hill repeat training I did was 6X2 minutes.

Any other suggestions?