I don't have a problem with heart rate or breathing - I am a rather slow runner - I go by how my knee is feeling whether I run uphill or not. It IS taking longer for it to start getting cranky That being said, what passes for hills on the mountain bike trails I am currently running are short and rolling. I know where the longest/steepest climbs are and I've not tried those trails yet. I will though, I will. And I won't be ashamed to walk them! I KNOW that I will have to pay much closer attention to HR and breathing. One of those hills on a very cool mountain bike trail has seen my HR > 155 WALKING it! The trail in that direction has a reputation. I never did attempt it on the bike as it was beyond my skill level and I knew that.

One thing about all my sports related injuries these past 5 years, I now listen to my body, it got the point across