High screen brightness and CPU/GPU intensive Apps can quickly deplete your iPad battery. Keeping the screen brightness at 30% or less can significantly improve battery autonomy - however, if the iPad is being used outdoors or in very bright ambient lighting, higher screen brightness may be unavoidable.
While your iPad's quoted battery autonomy is always "up to" - and is not an absolute. These figures will be in ideal conditions, which may not reflect achievable values in your typical usage. Speaking for myself, when used in similar conditions, my iPad Pro may realistically achieve five to seven hours before requiring full recharge.
Assuming that you are unable to carry and use your iPad's Power Adapter to recharge or power your iPad when the opportunity allows, consider investing in a decent-sized PowerBank (with a capacity of 20,000 mAh or greater). This will allow you to power your iPad for an extended period away from AC-mains power - while simulating charging your iPad's internal battery. Mophie (Zagg) and Anker offer well respected options.
When choosing a PowerBank, ensure that it supports USB PD (Power Delivery), this being the charging standard supported by Apple devices. Your iPad does not support other proprietary charging standards such as QC (Quick Charge); these will not charge efficiently, if at all.
You may be able to marginally improve battery autonomy by refining your iPad settings - as suggested by Ingo2711 - however, this is unlikely to make a significant difference for your described usage.
