This time of year means heat and humidity. Some places are worse than others, but much of the United States experiences it to some extent in July and August.
- Take a lighter/sportier bike to make the hill climbs a little easier (bring that Raleigh Sports instead of your DL-1 or your balloon tire cruiser, for example). You don't need to ride a 21st century bike with 20 speeds, but by the same token, don't force yourself to ride a bike where you're going to overwork.
- Brink a drink with you. Water is OK for shorter rides, but bring something with electrolytes for longer rides. A little sugar is OK, but don't overdo it on sweet drinks.
- On the other hand, drink in moderation and only gradually. Don't drink down half a bottle of Gatorade on your first break. Drinking too much all at once can be almost as bad as not drinking enough. Pace ride and pace your fluid intake to match your progress.
- Pick a route with more tree shade. My route runs through a wooded area, for about 2/3 of the run, which is great. Wooded areas that have shade throughout the day tend to stay substantially cooler.
- Ride in afternoon or evening. This is almost like a religious question - some people are "evening people" and some people are "morning people" when it comes to exercise. I'm an evening person. My wife is definitely a morning person (she claims only "weird people" want to exercise in the evening! Either way, only "mad dogs & Englishmen go out in the midday sun").
- Use a helmet with extra vent holes. There are some newer designs that are very light and let quite a bit of air through.
- Dress appropriately - light fabrics, light colors, shorts, etc.
- Always stop and take a break if you feel discomfort or distress - don't force yourself ahead if something feels wrong.
- Build yourself up: work up your ride distances and speeds gradually in such conditions. Don't force yourself into that "challenge ride" where you want to go 20% farther than usual. Distance, humidity, and heat all have a multiplying effect on the difficulty.
- Accept that your speed and progress might be a bit slower than usual. It's OK to go slower and take more breaks. Enjoy the ride more than trying to take a personal best.