Seems totally reasonable: I am confident in my ability to do this job now, but I'm less sure I'll be able to do it three or four years from now, so I will be se

Seems totally reasonable: I am confident in my ability to do this job now, but I'm less sure I'll be able to do it three or four years from now, so I will be seeing out this contract and won't sign a new one with you. A perfectly normal thing most working people do in their 60s.