Plenty of endurance athletes are relatively muscular. XC skiers and rowers, for example. You're not going to be successful at nordic skiing or rowing if you have no power in your upper body. I was around 165 at 5'11 when I was running varsity cross country in high school (and we didn't suck); there were several other fairly muscular guys on the team who were mid-varsity like me. Our fastest few runners did have the more typical endurance runner build.
I've lost endurance as I've gained weight, sure, but that's not a consequence of my muscle. It's because I stopped training the same way. Your cardio drops way off if you don't keep up your training intensity; I would notice it after as little as 2 weeks off back when I was a serious distance runner. Now I take a much more balanced approach to training. I want to be able to deadlift 500+ pounds and row a sub 7 minute 2000m*. If that means I'll never hit a 6:30 2000m, so be it.
*I can do neither right now. My 2km is in the high 7s somewhere and I can only deadlift in the mid 400s. Give me until October.
I've lost endurance as I've gained weight, sure, but that's not a consequence of my muscle. It's because I stopped training the same way. Your cardio drops way off if you don't keep up your training intensity; I would notice it after as little as 2 weeks off back when I was a serious distance runner. Now I take a much more balanced approach to training. I want to be able to deadlift 500+ pounds and row a sub 7 minute 2000m*. If that means I'll never hit a 6:30 2000m, so be it.
*I can do neither right now. My 2km is in the high 7s somewhere and I can only deadlift in the mid 400s. Give me until October.