Last weekend I went out for a ride and still felt great after about 35 miles. However, within the next few miles I went downhill very fast (no pun intended!). By the time I got to 40 miles my legs had gone. I assume I just ran out of energy. How much difference do energy bars / gels / drinks make? Obviously they'll help but is there a rule of thumb to give me an idea of the benefit they'll provide (eg + 10%)? How long do they take to take effect? Can anyone recommend any good ones? Many thanks.