Expiring after 50 miles even in silly cold sounds wrong to me.
( Unless you took 14 hours to do the 50 miles! )
A fully-charged edge 800 is good for about 14 hours in cool-ish conditions.
I rode a silly 200k in nearly 14 hours a month or so back, and it was bleating by the end. ( And so was I! )
I did not have it on the map display page, though; that may use more battery.
Also, it depends how much back-light you use.
14 hours is for using it as a track-recorder only, with no back-light use or map display or routing in use.
I've also used mine on a silly-cold commute, but only for 40 miles, where it then got a warm-up and then a further 40 miles in silly-cold. It's capacity was not noticably reduced compared to normal temperature on that route.
I have a TeckNet iEP390 11000mAh external battery pack which I use on longer rides, and it was not even off the 'full' mark after a 40-hour ride.