I took for granted that the OP would exercise some discretion in checking the weather as well as his or her own physical condition. (Note well the words "in good shape" in my message.)

Pinnacle Peak is hardly a mountain. Not being able to "hike" the Desert Botannical Garden (and yes, the convention and visitors bureau classifies this as a "hike," albeit an easy one) regardless of the temperature, would indicate that one is in poor physical condition.

By the way, if it is so dangerous, then why are these trails OPEN during the heat?