Can I manually input hr or to keep my exercise load up to date? I forgot my hr Strap on a 8 hr endurance ride and would like to have that ride represented in my exercise load Thank you

You can edit the activity and enter the avg and max HR values for the reference, but it won't have any impact on the exercise load - that's being measured only live in real-time. And also the values of the current power or speed, and the HRV and the respiration rate play a role, hence it cannot be calculated retrospectively just from the average HR.