5K with 305

Ran 5K this am with the 305. Tried to get in under 20 minutes, but around the two mile mark ran out of gas and had to walk. I noticed this came near the end of a long hill (rising about 100').

I noticed my heartrate was topping 180bps as started to feel bad at two miles.
The next time I will try to ignore time and keep my heart in the low-mid 170s.
Zone5 90-100% of max HR and I think it would be better to run mostly in zone 4.
Even with the walking I finished in 23:12 beating the 5K time I got in November.