Jay Leno, whose workaholic constitution has been the theme of numerous profiles about him, canceled the taping of the Tonight show Thursday and checked himself into a hospital for observation, his spokesman, Dick Guttman, said Thursday. Guttman, who would not identify Leno's illness or even the hospital where he was treated, said later that the talk show host was doing well and planned to return to work next week. The health scare must have raised the blood pressure of NBC executives who are planning a Leno-hosted variety hour to air nightly at 10 00 p.m. beginning this fall.