Map Pilot & Drone Battery Life Issues

Tried map pilot for the first time today with DJI P4P in a rural and wide open mountainous area to map our property of 50 acres. Top of our property is at 1200ft and the bottom is at 800ft. Used terrain aware and flew at 375ft with 80/75 overlap and auto exposure settings. It was a perfect even lit, overcast day with little to no wind. When planning the mission, it stated it would be a <15min flight and only use 1 battery.

Issues I had that I need explained:

- When I got on site, it showed that it could actually only complete 3/4 of the route due to too many points??

- 46 photos in (out of 200+) and the battery was down to 30%

- Flight speed was at 4mph

- Drone went into ATTI mode and drifted over the valley despite having a lock on 13 sats and had to be brought back with about 20% battery.

Should we just do this in smaller chunks? How would we mesh together multiple "missions?" Any insight or help to complete this is greatly appreciated. We really want to get the land mapped before the leaves are fully on.