58.5 y.o., lots of riding the last 5 years, lots of spin class time in year 3 and 4.

Thanks to mixing in MTBing I've gotten pretty confident tackling +15% grades on the road, normally getting over the first one not too winded and feeling pretty strong. Hills later down the road are a much different thing; although the bigger muscles are still good to go fatigue sets in while spinning up. If I stop halfway up and hydrate, shake the legs out then I'm good to go for 2 or 3 minutes, then they shut down again. After 3 or 4 hills, the legs don't want to climb at all; plenty of power and cadence on the flats but nothing in the legs for the grades. I've stopped, had something to eat, stretched the legs out, napped - doesn't matter, they're dead for the day. It's a very odd feeling to feel relatively fresh and strong during a ride but know your legs have no pop left. To compensate, I end up mashing or standing on whatever hills I have left to get over. I like standing, but not if it's the only option.

If you've dealt with this is it an age, nutrition, cadence training, technique or all the above deal? I am loving the hill challenge thing, but one and your done is unsatisfying. I'm think interval training at high cadence might be the cure, maybe mixing standing and spinning on every hill, especially the early ones. What do you think?