I did my first lap with the app today. I use a Samsung S8+, the power comes from a BAAS mini-USB, which only supplies 1A.
The brightness was permanently set to max, besides MRA I was running the Garmin Drive app (with Bluetooth connection to my Zumo XT) and an app to monitor the battery charge.
On the MRA app, route navigation and tracking were on all the time. The map settings were Theme=Light, Type=Normal, Building=Off.
I started with a battery charge of 80%, after two hours the battery was fully charged. So no problems with battery capacity at all.
Maybe the power consumption depends a lot on the smartphone model?