Apple Watch is running out of battery fast

To understand Apple Watch performance and its relation to your battery, see:

Apple Watch - Battery - Apple

Apple Watch Battery and Performance - Apple Support

“Battery life” is the amount of time your device runs before it needs to be recharged. “Battery lifespan” is the amount of time your battery lasts until it needs to be replaced. Maximize both and you’ll get the most out of your Apple devices, no matter which ones you own. See the "Tips for Apple Watch" section in:

Batteries - Maximizing Performance - Apple

If the battery drains too fast after an update, see all the replies to:

Update 6.2.1 Battery Draining Too Quickly… - Apple Community

FWIW, in some cases, unpairing the watch, completely erasing it, then pairing and restoring the backup will resolve the battery draining issue after an update.

Without knowing how long "very long" is, Apple Watch should last for between 12-18 hours depending on usage. For more info, go here: https://support.apple.com/en-us/HT210551