What you describe is very common for athletes who start too fast and do not have sufficient aerobic base coupled with muscular endurance training. A low heart rate with high effort indicates local muscular fatigue. This is just a guess because we do not know what you have done for training. In what intensity zone is 150-160? Chances are good that you need to start slower. It is much easier to make up ground and places in the later stages of races when others are fading and you can still be strong than it is to go out too hard and then be the one who fades away at the end.

More Aerobic Base and establish your zones carefully. Add ME training on top of a good aerobic base and then pace yourself a bit more.

Scott

Yes, it is possible that your uphill Z3 is different than your downhill/flat Z3. It is quite possible that local muscular fatigue from the downhills is have an effect on your uphill speed late in the race. I’ve seen this often. The way we have fixed this is with specific ME training. YOU might want to read this https://uphillathlete.com/ultrarunning-tor-des-geants-luke-nelson/. We used the gym ME workouts described near the end of this article. Luke did 13 ME workouts over 16 weeks.

It goes without saying that this ME work is most effective when layered on top of a substantial aerobic base. And you must maintain that base volume training during the ME period.

Scott

Yuri Verkhoshansky, who is the godfather of modern strength training methods wrote extensively about what he called Local Muscular Endurance in his book Special Strength Training Manual for Coaches. In another of his books Block Training for Middle Distance Runners he explains the experiments he conducted over 3 years with elite level Soviet middle distance runners. This is where I learned about the methods. Since 1992 I have used this method successfully on hundreds of athletes of all levels from strong amateurs to the very highest level including the Olympics.

Scott