My advice, carry lights every day--just part of your standard equipment.
I worry about the wind dying everytime I go out. Passwind is right, wind is fickle, especially late afternoon. It also frequently dies around noon--that happens when the winds are light, just ocean breezes.
When we are camping w/ typical 300# (near capacity) load, averaging 5 mph (8 km/hr) is pretty good. So, to do a 35 mile day (56 km) is a 7 hr trip. We have done it, but as our last major camp trip proved, that is atypical. On that trip, (link: http://www.hobiecat.com/forums/viewtopic.php?f=70&t=7276&p=134807#p134807
), the first day, we had no wind (pedaled 11 mi--17.7 km); second day, we had unfavorable winds and dense fog, and we made 4 miles (6.4 km). The 3rd day, we had strong winds on our nose and quit after 4 mi. The 4th day conditions got worse--very strong headwinds (20+ mph--32+ km/hr), and we aborted the trip.
If you are just going out for a day sail and you have favorable winds, a 20 mi day is reasonable. If winds are light & variable, save yourself some frustration and don't get too far from home base. If you have headwinds, your 10 mi (16 km) trip can become 20 mi (32 km) because you have to tack off the wind.