Another Endomodo user here. Was using the free app until the .99 cent Endo Pro sale.
I am using Endomondo too. If I know it will be a long ride, I turn on "airplane" mode on my smart phone. It does not attempt to connect to network, and thus saves the battery. For example, my old battery is good for more than 6 hours. When I turn the connection back on (airplane mode - off) the Endomondo uploads my route.