If you aren't seeing improvement as the season goes along review your training journal. It looks to me that your overal volume is there. Perhaps your intensity volume is too much.
Race pace will take you into the anerobic zone (frequently in my case) during a long race and it's important to have enough glycogen in the muscles in order to respond to those surges in pace/effort. Are you eating enough on the bike? It might be something as simple as a few extra GU packs early in the race or an extra energy drink at one of the feeds.
Lastly review your tactics. If you spend a lot of time in the wind at the front of the pack you are burning a lot of energy. Are you covering every break? If you race with the same guys every week you'll know who you need to mark and who you can let go. It takes a little luck to get into the right break.
It's not how long you were at the front, it's being in front at the line that counts.