Bereshit 27:1-4 – Yitzhak's fear of imminent death prior to wanting to bless Esav

Bereshit 27:1-4 records that Yitzhak told Esav that he wanted to bless him before he died. This fear of death is a little surprising since we know that Yitzhak lived at least another 20 years, as Yaakov saw Yitzhak after he had been away from home for 20 years, 35:27. Of course, there is no reason to believe that Yitzhak knew that he would live 20 more years, as who knows when they are going to die? Yet, was there a specific reason that led Yitzhak to fear his imminent death? 

Rashi (on 27:2) quotes from the Midrash that Yitzhak was 123 and thus he was within five years of his mother's death. (Rashi's calculation is based on the Midrash that Yaakov studied at the school of Ever for 14 years after he left home, see his comments on 25:17.) 

Radak (on 27:1) suggests that it was the onset of blindness that caused Yitzhak to fear that he would die shortly. 

There is another possibility. If Yaakov did not study at the school of Ever, then Yitzhak was 137 at the time when he wanted to bless Esav.   Yitzhak was 137 since immediately after Yaakov stole the blessings intended for Esav, Yaakov left for Paddan-aram, 27:43, and Yaakov was 77 when he came to Paddan-aram (47:9, 45:11, 41:46, 30:25; 130-9-30-14=77). We also know that Yitzhak was 60 when Yaakov was born, 25:26, which means that Yitzhak was 137 (60 + 77) when he wanted to bless Esav. 137 years was significant to Yitzhak since this was the age when Yitzhak's older half-brother Yishmael died, 25:17. Thus, when Yitzhak was 137, he feared that his death was imminent.