+1 for building more endurance. I've only been riding for about a year, but I came from an ultra running background, so there was a good amount of endurance to start. I've ridden 7,500 miles this year so far and should easily hit 10k for the year. On Sunday I did a century with over 11,000ft, including six Cat2 climbs, had no issues, and very minimal soreness after. I believe this is due to my training volume. Ultra running is the same way, it's all about time on your feet, or in this case, time in the saddle.