Another thing that may be obvious but I didn't see posted above...your bike must be in top operating condition. You don't want any gear shifting issues (especially if there are hills on the course), or brake pads rubbing on your rim, or a saddle that is too low. Also make sure tires are properly inflated.
Clipless pedals may help a bit with efficiency but on the other hand if you try to use them in a different way than what your muscles are used to it may cause added fatigue. Also if they're not aligned properly it could cause knee issues.