This from a brides point of view, I got married in June.
I'd say that the duties are not just about the speech! Our bestman was fab and it was an honour for us that he was our bestman.
He organised the stag weekend, in consultation with the groom, so the night/weekend was what he wanted. He stayed with the groom the night before, they got ready together and he drove them to the church. He also took control of any of the duties which needed to be done by the ushers. He basically made sure everything was ok before the ceremony,making sure that everyone were in their places when they should be etc.
After the ceremony, he helped out the photographer to get the relevent people into the right groups. He mingled with the guests and made them feel welcome and at ease.
He also made a speech, if you're nervous of this part, be short. There's nothing worse than a nervous speaker rambling on to fill time. (don't tell anything too embarrassing, think - would you want that story to be told about you in front of your Granny and any in-laws?!?!). The bride and groom that have asked you will surely give you pointers of what they want, we said that we didn't want cards to be read out, for example.
Once the meal and evening do got underway, he had one duty - to enjoy himself, he deserved it!!
It sounds like he had loads to do, which in a way he did, but it all flows on the day and he enjoyed it (I hope!) too. We made sure that he wasn't under any pressure by ensuring that he knew in advance what we wanted from our day and he then worked around that.
I know of bestmen who have been used literally like slaves, been expected to do the job of a wedding planner!
Try to relax and enjoy it, the best way to do that is make sure you know what they expect of you. It's no good arriving tipsy if they expect you to take full control, equally don't try to run their day if they want it to be