I would have the same reaction as you if I tried what you did without previous conditioning -- your reaction is completely normal. Especially in the hot of summer you need to slowly work up the length and strain on your rides. Either ease up on your pace, add more breaks, or shorten the rides until you adapt.
I would include electrolytes in your water. It also helps to have a heart rate monitor and you will know if you are over-exerting that way so you can ease up or take a break before you wipe yourself out.