I was thinking along those lines as well. The solution from @niketnilay is nice in the sense that it allows you to use the convert command, but the pre-processing looks quite complex to me. Having to perform multiple matches and then a double replace. Wouldn't be surprised if simply extracting minutes and seconds and calculating the seconds like you do turns out more efficient. At least it is a hell of a lot easier to read and that's worth a lot if you ask me 🙂
@arianf, you can use convert command with dur2sec() function, however, it expects the Duration time string in the following format: (1) With days included included d+HH:MM:SS (2) Without Days: HH:MM:SS.
So you would need to create a separate eval to format your Time Duration to above format first (the eval can be saved as Calculated Field and/or Macro for maintenance and scalability. If time duration is absolute the following expects lower time units will be also present (if not the conditions will change and more conditions will be required) i.e. If exact 1 hour duration is present it be be 1h 0m 1s in log.