Know your limitations:
Some of us are not in our 20's any longer. Keep that in mind when you start out.
Try to always hike with at least one other person - some of us may hike alone at times and that is not always a good idea.
Know your hiking partner. Go at your own pace because if you try to keep up with people that are way out of your league, you might end up broken in little pieces or worse.
Hike with someone that is at your level or slightly better. On an arbitrary scale of 1 to 10 and, say you rate yourself as a 4, try hiking with someone at a level 5. This will challenge you and help make you a better hiker.
If you are new to the desert, try starting in the cool spring with short hikes and work your way up to the longer ones and the heat. Take time to acclimate to the warmer and dryer heat. You might have been a great hiker in the northwest where it is nice and cool most of the time, but the desert and the heat is a different ball game. It would be similar situation to me going back east and trying to deal with 150% humidity.
Work up to some of the hikes by hiking established trails at first. Don't try the cross country stuff until you know what you are capable of doing. There are several good hiking books dealing with the trail system here in the greater Las Vegas area, and it is the same for most of the southwest.
Take advantage of shade whenever possible. When resting, try not to sit directly on the ground. It can be 10 to 30 degrees hotter on the ground.
If possible, rest at least 10 minutes every 60 minutes. Increase your resting time as it gets hotter or when you feel the need.
Keep hydrated with water. Do not drink alcohol or coffee as they will dehydrate you. A cold beer is NOT the answer to thirst.
Everyone should complete a basic First Aid and CPR course, if for no other reason than to be able to help their own family or friends in the case of an emergency.
On the average, a person tends to acclimate to the weather in the area that they are living in about 4 weeks. So, if you have just moved to the southwest deserts in the middle of summer, from say cooler northwest environment, you may want to give yourself a few weeks to get use to the heat before venturing out on desert hikes. Then it would be advisable to start with slow easy hikes on a well traveled trail and preferably not during the heat of the day. If you hike during the summer months - which is not recommended - start very early in the morning and finish up in the early morning and keep hydrated.
And, oh yes, Know Your Limitations.