When touring I can manage 15 mph with ease and so the distance I can travel depends on the amount of daylight as riding at dusk is a good way to become road kill. Going 140 miles in a day was not a problem during the summer months but it was not possible to do safely during the winter months.
Something I learned first hand is the affect of the heat waves coming off the pavement by mid day and the risk of heatstroke. Temperatures that did no phase me when out running cross country in the desert were deadly when out on the road with my bike. Hydration helps but our bodies are still limited in terms of how much heat they can dissipate with perspiration. The relative humidity affects our ability to perspire and maintain our core temperature at a safe level and when I lived in Dallas it was very much a limitation. A local marathon runner in the olympic trials that took place in the area died from heatstroke during the race.
Snacks like raisins help to keep up the blood sugar levels for the muscles and are easy to eat while riding. Bananas are another good food for fuel and electrolytes.